diff --git a/core/src/main/java/org/bitcoinj/core/Block.java b/core/src/main/java/org/bitcoinj/core/Block.java
index c52101029..ef87faf4e 100644
--- a/core/src/main/java/org/bitcoinj/core/Block.java
+++ b/core/src/main/java/org/bitcoinj/core/Block.java
@@ -560,7 +560,7 @@ public class Block extends Message {
         // Allow injection of a fake clock to allow unit testing.
         long currentTime = Utils.currentTimeSeconds();
         if (time > currentTime + ALLOWED_TIME_DRIFT)
-            throw new VerificationException(String.format("Block too far in future: %d vs %d", time, currentTime + ALLOWED_TIME_DRIFT));
+            throw new VerificationException(String.format(Locale.US, "Block too far in future: %d vs %d", time, currentTime + ALLOWED_TIME_DRIFT));
     }
 
     private void checkSigOps() throws VerificationException {
diff --git a/core/src/main/java/org/bitcoinj/core/Peer.java b/core/src/main/java/org/bitcoinj/core/Peer.java
index 8a934bbf7..c34c70a24 100644
--- a/core/src/main/java/org/bitcoinj/core/Peer.java
+++ b/core/src/main/java/org/bitcoinj/core/Peer.java
@@ -489,7 +489,7 @@ public class Peer extends PeerSocketHandler {
                 peerVersion,
                 vPeerVersionMessage.subVer,
                 vPeerVersionMessage.localServices,
-                String.format("%tF %tT", peerTime, peerTime),
+                String.format(Locale.US, "%tF %tT", peerTime, peerTime),
                 vPeerVersionMessage.bestHeight);
         // Now it's our turn ...
         // Send an ACK message stating we accept the peers protocol version.
diff --git a/core/src/main/java/org/bitcoinj/core/PeerGroup.java b/core/src/main/java/org/bitcoinj/core/PeerGroup.java
index 65fdc28bf..1857be9ff 100644
--- a/core/src/main/java/org/bitcoinj/core/PeerGroup.java
+++ b/core/src/main/java/org/bitcoinj/core/PeerGroup.java
@@ -1719,7 +1719,7 @@ public class PeerGroup implements TransactionBroadcaster {
                         for (long sample : samples) average += sample;
                         average /= samples.length;
 
-                        log.info(String.format("%d blocks/sec, %d tx/sec, %d pre-filtered tx/sec, avg/last %.2f/%.2f kilobytes per sec (stall threshold <%.2f KB/sec for %d seconds)",
+                        log.info(String.format(Locale.US, "%d blocks/sec, %d tx/sec, %d pre-filtered tx/sec, avg/last %.2f/%.2f kilobytes per sec (stall threshold <%.2f KB/sec for %d seconds)",
                                 blocksInLastSecond, txnsInLastSecond, origTxnsInLastSecond, average / 1024.0, bytesInLastSecond / 1024.0,
                                 minSpeedBytesPerSec / 1024.0, samples.length));
 
@@ -1736,7 +1736,7 @@ public class PeerGroup implements TransactionBroadcaster {
                                 log.warn("This network seems to be slower than the requested stall threshold - won't do stall disconnects any more.");
                             } else {
                                 Peer peer = getDownloadPeer();
-                                log.warn(String.format("Chain download stalled: received %.2f KB/sec for %d seconds, require average of %.2f KB/sec, disconnecting %s", average / 1024.0, samples.length, minSpeedBytesPerSec / 1024.0, peer));
+                                log.warn(String.format(Locale.US, "Chain download stalled: received %.2f KB/sec for %d seconds, require average of %.2f KB/sec, disconnecting %s", average / 1024.0, samples.length, minSpeedBytesPerSec / 1024.0, peer));
                                 peer.close();
                                 // Reset the sample buffer and give the next peer time to get going.
                                 samples = null;
@@ -1746,7 +1746,7 @@ public class PeerGroup implements TransactionBroadcaster {
                     } else {
                         warmupSeconds--;
                         if (bytesInLastSecond > 0)
-                            log.info(String.format("%d blocks/sec, %d tx/sec, %d pre-filtered tx/sec, last %.2f kilobytes per sec",
+                            log.info(String.format(Locale.US, "%d blocks/sec, %d tx/sec, %d pre-filtered tx/sec, last %.2f kilobytes per sec",
                                     blocksInLastSecond, txnsInLastSecond, origTxnsInLastSecond, bytesInLastSecond / 1024.0));
                     }
                 }
diff --git a/core/src/main/java/org/bitcoinj/core/RejectMessage.java b/core/src/main/java/org/bitcoinj/core/RejectMessage.java
index f09566635..cae869622 100644
--- a/core/src/main/java/org/bitcoinj/core/RejectMessage.java
+++ b/core/src/main/java/org/bitcoinj/core/RejectMessage.java
@@ -20,6 +20,7 @@ package org.bitcoinj.core;
 import com.google.common.base.Objects;
 import java.io.IOException;
 import java.io.OutputStream;
+import java.util.Locale;
 
 /**
  * A message sent by nodes when a message we sent was rejected (ie a transaction had too little fee/was invalid/etc)
@@ -145,7 +146,7 @@ public class RejectMessage extends Message {
     @Override
     public String toString() {
         Sha256Hash hash = getRejectedObjectHash();
-        return String.format("Reject: %s %s for reason '%s' (%d)", getRejectedMessage(),
+        return String.format(Locale.US, "Reject: %s %s for reason '%s' (%d)", getRejectedMessage(),
             hash != null ? hash : "", getReasonString(), getReasonCode().code);
     }
 
diff --git a/core/src/main/java/org/bitcoinj/core/StoredBlock.java b/core/src/main/java/org/bitcoinj/core/StoredBlock.java
index 156de4839..4bc815300 100644
--- a/core/src/main/java/org/bitcoinj/core/StoredBlock.java
+++ b/core/src/main/java/org/bitcoinj/core/StoredBlock.java
@@ -22,6 +22,7 @@ import com.google.common.base.Objects;
 
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
+import java.util.Locale;
 
 import static com.google.common.base.Preconditions.checkState;
 
@@ -143,7 +144,7 @@ public class StoredBlock {
 
     @Override
     public String toString() {
-        return String.format("Block %s at height %d: %s",
+        return String.format(Locale.US, "Block %s at height %d: %s",
                 getHeader().getHashAsString(), getHeight(), getHeader().toString());
     }
 }
diff --git a/core/src/main/java/org/bitcoinj/core/Transaction.java b/core/src/main/java/org/bitcoinj/core/Transaction.java
index 68c6e9199..62435563e 100644
--- a/core/src/main/java/org/bitcoinj/core/Transaction.java
+++ b/core/src/main/java/org/bitcoinj/core/Transaction.java
@@ -636,7 +636,7 @@ public class Transaction extends ChildMessage {
     public String toString(@Nullable AbstractBlockChain chain) {
         // Basic info about the tx.
         StringBuilder s = new StringBuilder();
-        s.append(String.format("  %s: %s%n", getHashAsString(), getConfidence()));
+        s.append(String.format(Locale.US, "  %s: %s%n", getHashAsString(), getConfidence()));
         if (isTimeLocked()) {
             String time;
             if (lockTime < LOCKTIME_THRESHOLD) {
@@ -648,10 +648,10 @@ public class Transaction extends ChildMessage {
             } else {
                 time = new Date(lockTime*1000).toString();
             }
-            s.append(String.format("  time locked until %s%n", time));
+            s.append(String.format(Locale.US, "  time locked until %s%n", time));
         }
         if (inputs.size() == 0) {
-            s.append(String.format("  INCOMPLETE: No inputs!%n"));
+            s.append(String.format(Locale.US, "  INCOMPLETE: No inputs!%n"));
             return s.toString();
         }
         if (isCoinBase()) {
@@ -692,7 +692,7 @@ public class Transaction extends ChildMessage {
             } catch (Exception e) {
                 s.append("[exception: ").append(e.getMessage()).append("]");
             }
-            s.append(String.format("%n"));
+            s.append(String.format(Locale.US, "%n"));
         }
         for (TransactionOutput out : outputs) {
             s.append("     ");
@@ -712,13 +712,13 @@ public class Transaction extends ChildMessage {
             } catch (Exception e) {
                 s.append("[exception: ").append(e.getMessage()).append("]");
             }
-            s.append(String.format("%n"));
+            s.append(String.format(Locale.US, "%n"));
         }
         Coin fee = getFee();
         if (fee != null)
-            s.append("     fee  ").append(fee.toFriendlyString()).append(String.format("%n"));
+            s.append("     fee  ").append(fee.toFriendlyString()).append(String.format(Locale.US, "%n"));
         if (purpose != null)
-            s.append("     prps ").append(purpose).append(String.format("%n"));
+            s.append("     prps ").append(purpose).append(String.format(Locale.US, "%n"));
         return s.toString();
     }
 
diff --git a/core/src/main/java/org/bitcoinj/core/TransactionConfidence.java b/core/src/main/java/org/bitcoinj/core/TransactionConfidence.java
index 0b15f3eb0..19779f423 100644
--- a/core/src/main/java/org/bitcoinj/core/TransactionConfidence.java
+++ b/core/src/main/java/org/bitcoinj/core/TransactionConfidence.java
@@ -336,7 +336,7 @@ public class TransactionConfidence {
                 builder.append("In conflict.");
                 break;
             case BUILDING:
-                builder.append(String.format("Appeared in best chain at height %d, depth %d.",
+                builder.append(String.format(Locale.US, "Appeared in best chain at height %d, depth %d.",
                         getAppearedAtChainHeight(), getDepthInBlocks()));
                 break;
         }
diff --git a/core/src/main/java/org/bitcoinj/core/UTXO.java b/core/src/main/java/org/bitcoinj/core/UTXO.java
index 16e45a25c..427a6bfc8 100644
--- a/core/src/main/java/org/bitcoinj/core/UTXO.java
+++ b/core/src/main/java/org/bitcoinj/core/UTXO.java
@@ -21,6 +21,7 @@ import com.google.common.base.Objects;
 
 import java.io.*;
 import java.math.*;
+import java.util.Locale;
 
 // TODO: Fix this class: should not talk about addresses, height should be optional/support mempool height etc
 
@@ -156,7 +157,7 @@ public class UTXO {
 
     @Override
     public String toString() {
-        return String.format("Stored TxOut of %s (%s:%d)", value.toFriendlyString(), hash, index);
+        return String.format(Locale.US, "Stored TxOut of %s (%s:%d)", value.toFriendlyString(), hash, index);
     }
 
     @Override
diff --git a/core/src/main/java/org/bitcoinj/core/VersionMessage.java b/core/src/main/java/org/bitcoinj/core/VersionMessage.java
index 8c019278d..1f2c034e3 100644
--- a/core/src/main/java/org/bitcoinj/core/VersionMessage.java
+++ b/core/src/main/java/org/bitcoinj/core/VersionMessage.java
@@ -22,6 +22,7 @@ import java.io.IOException;
 import java.io.OutputStream;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
+import java.util.Locale;
 
 /**
  * A VersionMessage holds information exchanged during connection setup with another peer. Most of the fields are not
@@ -259,9 +260,9 @@ public class VersionMessage extends Message {
         checkSubVerComponent(version);
         if (comments != null) {
             checkSubVerComponent(comments);
-            subVer = subVer.concat(String.format("%s:%s(%s)/", name, version, comments));
+            subVer = subVer.concat(String.format(Locale.US, "%s:%s(%s)/", name, version, comments));
         } else {
-            subVer = subVer.concat(String.format("%s:%s/", name, version));
+            subVer = subVer.concat(String.format(Locale.US, "%s:%s/", name, version));
         }
     }
 
diff --git a/core/src/main/java/org/bitcoinj/core/Wallet.java b/core/src/main/java/org/bitcoinj/core/Wallet.java
index 01ee58cca..684828d8f 100644
--- a/core/src/main/java/org/bitcoinj/core/Wallet.java
+++ b/core/src/main/java/org/bitcoinj/core/Wallet.java
@@ -1617,7 +1617,7 @@ public class Wallet extends BaseTaggableObject
             Coin valueSentToMe = tx.getValueSentToMe(this);
             Coin valueSentFromMe = tx.getValueSentFromMe(this);
             if (log.isInfoEnabled()) {
-                log.info(String.format("Received a pending transaction %s that spends %s from our own wallet," +
+                log.info(String.format(Locale.US, "Received a pending transaction %s that spends %s from our own wallet," +
                         " and sends us %s", tx.getHashAsString(), valueSentFromMe.toFriendlyString(),
                         valueSentToMe.toFriendlyString()));
             }
@@ -2943,19 +2943,19 @@ public class Wallet extends BaseTaggableObject
             StringBuilder builder = new StringBuilder();
             Coin estimatedBalance = getBalance(BalanceType.ESTIMATED);
             Coin availableBalance = getBalance(BalanceType.AVAILABLE_SPENDABLE);
-            builder.append(String.format("Wallet containing %s BTC (spendable: %s BTC) in:%n",
+            builder.append(String.format(Locale.US, "Wallet containing %s BTC (spendable: %s BTC) in:%n",
                     estimatedBalance.toPlainString(), availableBalance.toPlainString()));
-            builder.append(String.format("  %d pending transactions%n", pending.size()));
-            builder.append(String.format("  %d unspent transactions%n", unspent.size()));
-            builder.append(String.format("  %d spent transactions%n", spent.size()));
-            builder.append(String.format("  %d dead transactions%n", dead.size()));
+            builder.append(String.format(Locale.US, "  %d pending transactions%n", pending.size()));
+            builder.append(String.format(Locale.US, "  %d unspent transactions%n", unspent.size()));
+            builder.append(String.format(Locale.US, "  %d spent transactions%n", spent.size()));
+            builder.append(String.format(Locale.US, "  %d dead transactions%n", dead.size()));
             final Date lastBlockSeenTime = getLastBlockSeenTime();
             final String lastBlockSeenTimeStr = lastBlockSeenTime == null ? "time unknown" : lastBlockSeenTime.toString();
-            builder.append(String.format("Last seen best block: %d (%s): %s%n",
+            builder.append(String.format(Locale.US, "Last seen best block: %d (%s): %s%n",
                     getLastBlockSeenHeight(), lastBlockSeenTimeStr, getLastBlockSeenHash()));
             final KeyCrypter crypter = keyChainGroup.getKeyCrypter();
             if (crypter != null)
-                builder.append(String.format("Encryption: %s%n", crypter));
+                builder.append(String.format(Locale.US, "Encryption: %s%n", crypter));
             if (isWatching())
                 builder.append("Wallet is watching.\n");
 
@@ -2963,7 +2963,7 @@ public class Wallet extends BaseTaggableObject
             builder.append("\nKeys:\n");
             final long keyRotationTime = vKeyRotationTimestamp * 1000;
             if (keyRotationTime > 0)
-                builder.append(String.format("Key rotation time: %s\n", Utils.dateTimeFormat(keyRotationTime)));
+                builder.append(String.format(Locale.US, "Key rotation time: %s\n", Utils.dateTimeFormat(keyRotationTime)));
             builder.append(keyChainGroup.toString(includePrivateKeys));
 
             if (!watchedScripts.isEmpty()) {
diff --git a/core/src/main/java/org/bitcoinj/core/listeners/DownloadProgressTracker.java b/core/src/main/java/org/bitcoinj/core/listeners/DownloadProgressTracker.java
index cd7ded0a7..24c1eb270 100644
--- a/core/src/main/java/org/bitcoinj/core/listeners/DownloadProgressTracker.java
+++ b/core/src/main/java/org/bitcoinj/core/listeners/DownloadProgressTracker.java
@@ -28,6 +28,7 @@ import org.slf4j.LoggerFactory;
 
 import javax.annotation.*;
 import java.util.Date;
+import java.util.Locale;
 import java.util.concurrent.ExecutionException;
 
 /**
@@ -86,7 +87,7 @@ public class DownloadProgressTracker extends AbstractPeerDataEventListener {
      * @param date the date of the last block downloaded
      */
     protected void progress(double pct, int blocksSoFar, Date date) {
-        log.info(String.format("Chain download %d%% done with %d blocks to go, block date %s", (int) pct, blocksSoFar,
+        log.info(String.format(Locale.US, "Chain download %d%% done with %d blocks to go, block date %s", (int) pct, blocksSoFar,
                 Utils.dateTimeFormat(date)));
     }
 
diff --git a/core/src/main/java/org/bitcoinj/crypto/ChildNumber.java b/core/src/main/java/org/bitcoinj/crypto/ChildNumber.java
index 0540b21ed..8a42e1c11 100644
--- a/core/src/main/java/org/bitcoinj/crypto/ChildNumber.java
+++ b/core/src/main/java/org/bitcoinj/crypto/ChildNumber.java
@@ -16,6 +16,8 @@
 
 package org.bitcoinj.crypto;
 
+import java.util.Locale;
+
 import com.google.common.primitives.Ints;
 
 /**
@@ -73,7 +75,7 @@ public class ChildNumber implements Comparable<ChildNumber> {
 
     @Override
     public String toString() {
-        return String.format("%d%s", num(), isHardened() ? "H" : "");
+        return String.format(Locale.US, "%d%s", num(), isHardened() ? "H" : "");
     }
 
     @Override
diff --git a/core/src/main/java/org/bitcoinj/crypto/DeterministicHierarchy.java b/core/src/main/java/org/bitcoinj/crypto/DeterministicHierarchy.java
index 8404f2905..eba119fd7 100644
--- a/core/src/main/java/org/bitcoinj/crypto/DeterministicHierarchy.java
+++ b/core/src/main/java/org/bitcoinj/crypto/DeterministicHierarchy.java
@@ -20,6 +20,7 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Maps;
 
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 
 import static com.google.common.base.Preconditions.checkArgument;
@@ -85,7 +86,7 @@ public class DeterministicHierarchy {
                 : ImmutableList.copyOf(path);
         if (!keys.containsKey(absolutePath)) {
             if (!create)
-                throw new IllegalArgumentException(String.format("No key found for %s path %s.",
+                throw new IllegalArgumentException(String.format(Locale.US, "No key found for %s path %s.",
                     relativePath ? "relative" : "absolute", HDUtils.formatPath(path)));
             checkArgument(absolutePath.size() > 0, "Can't derive the master key: nothing to derive from.");
             DeterministicKey parent = get(absolutePath.subList(0, absolutePath.size() - 1), false, true);
diff --git a/core/src/main/java/org/bitcoinj/protocols/channels/PaymentChannelServerState.java b/core/src/main/java/org/bitcoinj/protocols/channels/PaymentChannelServerState.java
index d65c89330..110c6d1e8 100644
--- a/core/src/main/java/org/bitcoinj/protocols/channels/PaymentChannelServerState.java
+++ b/core/src/main/java/org/bitcoinj/protocols/channels/PaymentChannelServerState.java
@@ -31,6 +31,7 @@ import org.slf4j.LoggerFactory;
 
 import javax.annotation.Nullable;
 import java.util.Arrays;
+import java.util.Locale;
 
 import static com.google.common.base.Preconditions.*;
 
@@ -399,7 +400,7 @@ public class PaymentChannelServerState {
             feePaidForPayment = req.tx.getFee();
             log.info("Calculated fee is {}", feePaidForPayment);
             if (feePaidForPayment.compareTo(bestValueToMe) > 0) {
-                final String msg = String.format("Had to pay more in fees (%s) than the channel was worth (%s)",
+                final String msg = String.format(Locale.US, "Had to pay more in fees (%s) than the channel was worth (%s)",
                         feePaidForPayment, bestValueToMe);
                 throw new InsufficientMoneyException(feePaidForPayment.subtract(bestValueToMe), msg);
             }
diff --git a/core/src/main/java/org/bitcoinj/protocols/channels/StoredPaymentChannelClientStates.java b/core/src/main/java/org/bitcoinj/protocols/channels/StoredPaymentChannelClientStates.java
index d79adf5ad..544d0f12d 100644
--- a/core/src/main/java/org/bitcoinj/protocols/channels/StoredPaymentChannelClientStates.java
+++ b/core/src/main/java/org/bitcoinj/protocols/channels/StoredPaymentChannelClientStates.java
@@ -28,6 +28,7 @@ import org.slf4j.LoggerFactory;
 
 import javax.annotation.Nullable;
 import java.util.Date;
+import java.util.Locale;
 import java.util.Set;
 import java.util.Timer;
 import java.util.TimerTask;
@@ -371,9 +372,9 @@ class StoredClientChannel {
 
     @Override
     public String toString() {
-        final String newline = String.format("%n");
+        final String newline = String.format(Locale.US, "%n");
         final String closeStr = close == null ? "still open" : close.toString().replaceAll(newline, newline + "   ");
-        return String.format("Stored client channel for server ID %s (%s)%n" +
+        return String.format(Locale.US, "Stored client channel for server ID %s (%s)%n" +
                 "    Key:         %s%n" +
                 "    Value left:  %s%n" +
                 "    Refund fees: %s%n" +
diff --git a/core/src/main/java/org/bitcoinj/protocols/channels/StoredServerChannel.java b/core/src/main/java/org/bitcoinj/protocols/channels/StoredServerChannel.java
index 6f168b6f9..674f8813e 100644
--- a/core/src/main/java/org/bitcoinj/protocols/channels/StoredServerChannel.java
+++ b/core/src/main/java/org/bitcoinj/protocols/channels/StoredServerChannel.java
@@ -20,6 +20,7 @@ import org.bitcoinj.core.*;
 
 import javax.annotation.Nullable;
 import java.util.Date;
+import java.util.Locale;
 
 import static com.google.common.base.Preconditions.checkArgument;
 
@@ -103,8 +104,8 @@ public class StoredServerChannel {
 
     @Override
     public synchronized String toString() {
-        final String newline = String.format("%n");
-        return String.format("Stored server channel (%s)%n" +
+        final String newline = String.format(Locale.US, "%n");
+        return String.format(Locale.US, "Stored server channel (%s)%n" +
                 "    Key:           %s%n" +
                 "    Value to me:   %s%n" +
                 "    Client output: %s%n" +
diff --git a/core/src/main/java/org/bitcoinj/store/DatabaseFullPrunedBlockStore.java b/core/src/main/java/org/bitcoinj/store/DatabaseFullPrunedBlockStore.java
index 30e5f150b..3c79dc3c1 100644
--- a/core/src/main/java/org/bitcoinj/store/DatabaseFullPrunedBlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/DatabaseFullPrunedBlockStore.java
@@ -1215,7 +1215,7 @@ public abstract class DatabaseFullPrunedBlockStore implements FullPrunedBlockSto
             count++;
         }
         rs.close();
-        System.out.printf("Settings size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
+        System.out.printf(Locale.US, "Settings size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
 
         totalSize += size; size = 0; count = 0;
         rs = s.executeQuery(getSelectHeadersDumpSQL());
@@ -1227,7 +1227,7 @@ public abstract class DatabaseFullPrunedBlockStore implements FullPrunedBlockSto
             count++;
         }
         rs.close();
-        System.out.printf("Headers size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
+        System.out.printf(Locale.US, "Headers size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
 
         totalSize += size; size = 0; count = 0;
         rs = s.executeQuery(getSelectUndoableblocksDumpSQL());
@@ -1244,7 +1244,7 @@ public abstract class DatabaseFullPrunedBlockStore implements FullPrunedBlockSto
             count++;
         }
         rs.close();
-        System.out.printf("Undoable Blocks size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
+        System.out.printf(Locale.US, "Undoable Blocks size: %d, count: %d, average size: %f%n", size, count, (double)size/count);
 
         totalSize += size; size = 0; count = 0;
         long scriptSize = 0;
@@ -1259,7 +1259,7 @@ public abstract class DatabaseFullPrunedBlockStore implements FullPrunedBlockSto
             count++;
         }
         rs.close();
-        System.out.printf("Open Outputs size: %d, count: %d, average size: %f, average script size: %f (%d in id indexes)%n",
+        System.out.printf(Locale.US, "Open Outputs size: %d, count: %d, average size: %f, average script size: %f (%d in id indexes)%n",
                 size, count, (double)size/count, (double)scriptSize/count, count * 8);
 
         totalSize += size;
diff --git a/core/src/main/java/org/bitcoinj/store/WalletProtobufSerializer.java b/core/src/main/java/org/bitcoinj/store/WalletProtobufSerializer.java
index b547c1193..ccf714e9b 100644
--- a/core/src/main/java/org/bitcoinj/store/WalletProtobufSerializer.java
+++ b/core/src/main/java/org/bitcoinj/store/WalletProtobufSerializer.java
@@ -666,7 +666,7 @@ public class WalletProtobufSerializer {
         // Transaction should now be complete.
         Sha256Hash protoHash = byteStringToHash(txProto.getHash());
         if (!tx.getHash().equals(protoHash))
-            throw new UnreadableWalletException(String.format("Transaction did not deserialize completely: %s vs %s", tx.getHash(), protoHash));
+            throw new UnreadableWalletException(String.format(Locale.US, "Transaction did not deserialize completely: %s vs %s", tx.getHash(), protoHash));
         if (txMap.containsKey(txProto.getHash()))
             throw new UnreadableWalletException("Wallet contained duplicate transaction " + byteStringToHash(txProto.getHash()));
         txMap.put(txProto.getHash(), tx);
@@ -699,7 +699,7 @@ public class WalletProtobufSerializer {
                 final ByteString spentByTransactionHash = transactionOutput.getSpentByTransactionHash();
                 Transaction spendingTx = txMap.get(spentByTransactionHash);
                 if (spendingTx == null) {
-                    throw new UnreadableWalletException(String.format("Could not connect %s to %s",
+                    throw new UnreadableWalletException(String.format(Locale.US, "Could not connect %s to %s",
                             tx.getHashAsString(), byteStringToHash(spentByTransactionHash)));
                 }
                 final int spendingIndex = transactionOutput.getSpentByTransactionIndex();
diff --git a/core/src/main/java/org/bitcoinj/uri/BitcoinURI.java b/core/src/main/java/org/bitcoinj/uri/BitcoinURI.java
index 18f1cbe36..59b3f62d0 100644
--- a/core/src/main/java/org/bitcoinj/uri/BitcoinURI.java
+++ b/core/src/main/java/org/bitcoinj/uri/BitcoinURI.java
@@ -225,9 +225,9 @@ public class BitcoinURI {
                         throw new ArithmeticException("Negative coins specified");
                     putWithValidation(FIELD_AMOUNT, amount);
                 } catch (IllegalArgumentException e) {
-                    throw new OptionalFieldValidationException(String.format("'%s' is not a valid amount", valueToken), e);
+                    throw new OptionalFieldValidationException(String.format(Locale.US, "'%s' is not a valid amount", valueToken), e);
                 } catch (ArithmeticException e) {
-                    throw new OptionalFieldValidationException(String.format("'%s' has too many decimal places", valueToken), e);
+                    throw new OptionalFieldValidationException(String.format(Locale.US, "'%s' has too many decimal places", valueToken), e);
                 }
             } else {
                 if (nameToken.startsWith("req-")) {
@@ -257,7 +257,7 @@ public class BitcoinURI {
      */
     private void putWithValidation(String key, Object value) throws BitcoinURIParseException {
         if (parameterMap.containsKey(key)) {
-            throw new BitcoinURIParseException(String.format("'%s' is duplicated, URI is invalid", key));
+            throw new BitcoinURIParseException(String.format(Locale.US, "'%s' is duplicated, URI is invalid", key));
         } else {
             parameterMap.put(key, value);
         }
diff --git a/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java b/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
index da507c71f..77869e796 100644
--- a/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
+++ b/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
@@ -28,6 +28,7 @@ import java.io.IOException;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Locale;
 import java.util.NoSuchElementException;
 
 /**
@@ -62,7 +63,7 @@ public class BlockFileLoader implements Iterable<Block>, Iterator<Block> {
         
         List<File> list = new LinkedList<File>();
         for (int i = 0; true; i++) {
-            File file = new File(defaultDataDir + String.format("blk%05d.dat", i));
+            File file = new File(defaultDataDir + String.format(Locale.US, "blk%05d.dat", i));
             if (!file.exists())
                 break;
             list.add(file);
diff --git a/core/src/main/java/org/bitcoinj/wallet/DeterministicKeyChain.java b/core/src/main/java/org/bitcoinj/wallet/DeterministicKeyChain.java
index 113636c78..a4aaf7f8e 100644
--- a/core/src/main/java/org/bitcoinj/wallet/DeterministicKeyChain.java
+++ b/core/src/main/java/org/bitcoinj/wallet/DeterministicKeyChain.java
@@ -528,7 +528,7 @@ public class DeterministicKeyChain implements EncryptableKeyChain {
         byte[] rederived = HDKeyDerivation.deriveChildKeyBytesFromPublic(parent, k.getChildNumber(), HDKeyDerivation.PublicDeriveMode.WITH_INVERSION).keyBytes;
         byte[] actual = k.getPubKey();
         if (!Arrays.equals(rederived, actual))
-            throw new IllegalStateException(String.format("Bit-flip check failed: %s vs %s", Arrays.toString(rederived), Arrays.toString(actual)));
+            throw new IllegalStateException(String.format(Locale.US, "Bit-flip check failed: %s vs %s", Arrays.toString(rederived), Arrays.toString(actual)));
     }
 
     private void addToBasicChain(DeterministicKey key) {
@@ -1331,19 +1331,19 @@ public class DeterministicKeyChain implements EncryptableKeyChain {
         final StringBuilder builder2 = new StringBuilder();
         if (seed != null) {
             if (seed.isEncrypted()) {
-                builder2.append(String.format("Seed is encrypted%n"));
+                builder2.append(String.format(Locale.US, "Seed is encrypted%n"));
             } else if (includePrivateKeys) {
                 final List<String> words = seed.getMnemonicCode();
                 builder2.append(
-                        String.format("Seed as words: %s%nSeed as hex:   %s%n", Utils.join(words),
+                        String.format(Locale.US, "Seed as words: %s%nSeed as hex:   %s%n", Utils.join(words),
                                 seed.toHexString())
                 );
             }
-            builder2.append(String.format("Seed birthday: %d  [%s]%n", seed.getCreationTimeSeconds(),
+            builder2.append(String.format(Locale.US, "Seed birthday: %d  [%s]%n", seed.getCreationTimeSeconds(),
                     Utils.dateTimeFormat(seed.getCreationTimeSeconds() * 1000)));
         }
         final DeterministicKey watchingKey = getWatchingKey();
-        builder2.append(String.format("Key to watch:  %s%n", watchingKey.serializePubB58(params)));
+        builder2.append(String.format(Locale.US, "Key to watch:  %s%n", watchingKey.serializePubB58(params)));
         formatAddresses(includePrivateKeys, params, builder2);
         return builder2.toString();
     }
diff --git a/core/src/main/java/org/bitcoinj/wallet/KeyChainGroup.java b/core/src/main/java/org/bitcoinj/wallet/KeyChainGroup.java
index 7d97fa736..f9f5768d0 100644
--- a/core/src/main/java/org/bitcoinj/wallet/KeyChainGroup.java
+++ b/core/src/main/java/org/bitcoinj/wallet/KeyChainGroup.java
@@ -808,7 +808,7 @@ public class KeyChainGroup implements KeyBag {
         for (DeterministicKeyChain chain : chains) {
             chainStrs.add(chain.toString(includePrivateKeys, params));
         }
-        builder.append(Joiner.on(String.format("%n")).join(chainStrs));
+        builder.append(Joiner.on(String.format(Locale.US, "%n")).join(chainStrs));
         return builder.toString();
     }
 
diff --git a/core/src/main/java/org/bitcoinj/wallet/MarriedKeyChain.java b/core/src/main/java/org/bitcoinj/wallet/MarriedKeyChain.java
index e3d9562a2..5c50b58f1 100644
--- a/core/src/main/java/org/bitcoinj/wallet/MarriedKeyChain.java
+++ b/core/src/main/java/org/bitcoinj/wallet/MarriedKeyChain.java
@@ -32,6 +32,7 @@ import org.bitcoinj.script.ScriptBuilder;
 import java.security.SecureRandom;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 
 import javax.annotation.Nullable;
@@ -238,9 +239,9 @@ public class MarriedKeyChain extends DeterministicKeyChain {
     @Override
     protected void formatAddresses(boolean includePrivateKeys, NetworkParameters params, StringBuilder builder2) {
         for (DeterministicKeyChain followingChain : followingKeyChains) {
-            builder2.append(String.format("Following chain:  %s%n", followingChain.getWatchingKey().serializePubB58(params)));
+            builder2.append(String.format(Locale.US, "Following chain:  %s%n", followingChain.getWatchingKey().serializePubB58(params)));
         }
-        builder2.append(String.format("%n"));
+        builder2.append(String.format(Locale.US, "%n"));
         for (RedeemData redeemData : marriedKeysRedeemData.values())
             formatScript(ScriptBuilder.createP2SHOutputScript(redeemData.redeemScript), builder2, params);
     }
diff --git a/core/src/test/java/org/bitcoinj/crypto/BIP32Test.java b/core/src/test/java/org/bitcoinj/crypto/BIP32Test.java
index 58c7ef8df..a7ec63283 100644
--- a/core/src/test/java/org/bitcoinj/crypto/BIP32Test.java
+++ b/core/src/test/java/org/bitcoinj/crypto/BIP32Test.java
@@ -29,6 +29,7 @@ import org.slf4j.LoggerFactory;
 
 import java.util.Arrays;
 import java.util.List;
+import java.util.Locale;
 
 import static org.bitcoinj.core.Utils.HEX;
 import static org.junit.Assert.assertEquals;
@@ -137,7 +138,7 @@ public class BIP32Test {
         for (int i = 0; i < tv.derived.size(); i++) {
             HDWTestVector.DerivedTestCase tc = tv.derived.get(i);
             log.info("{}", tc.name);
-            assertEquals(tc.name, String.format("Test%d %s", testCase + 1, tc.getPathDescription()));
+            assertEquals(tc.name, String.format(Locale.US, "Test%d %s", testCase + 1, tc.getPathDescription()));
             int depth = tc.path.length - 1;
             DeterministicKey ehkey = dh.deriveChild(Arrays.asList(tc.path).subList(0, depth), false, true, tc.path[depth]);
             assertEquals(testEncode(tc.priv), testEncode(ehkey.serializePrivB58(params)));