diff --git a/src/com/fsck/k9/activity/MessageCompose.java b/src/com/fsck/k9/activity/MessageCompose.java
index 8c8eaa0c6..6ea5c91a6 100644
--- a/src/com/fsck/k9/activity/MessageCompose.java
+++ b/src/com/fsck/k9/activity/MessageCompose.java
@@ -1565,7 +1565,7 @@ public class MessageCompose extends K9Activity implements OnClickListener,
              *  title*2*=%2A%2A%2Afun%2A%2A%2A%20
              *  title*3="isn't it!"
              */
-            bp.addHeader(MimeHeader.HEADER_CONTENT_DISPOSITION, String.format(
+            bp.addHeader(MimeHeader.HEADER_CONTENT_DISPOSITION, String.format(Locale.US,
                              "attachment;\r\n filename=\"%s\";\r\n size=%d",
                              attachment.name, attachment.size));
 
diff --git a/src/com/fsck/k9/activity/setup/AccountSetupBasics.java b/src/com/fsck/k9/activity/setup/AccountSetupBasics.java
index 94ba4cf5b..421e56368 100644
--- a/src/com/fsck/k9/activity/setup/AccountSetupBasics.java
+++ b/src/com/fsck/k9/activity/setup/AccountSetupBasics.java
@@ -24,6 +24,7 @@ import java.io.UnsupportedEncodingException;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.URLEncoder;
+import java.util.Locale;
 
 /**
  * Prompts the user for the email address and password.
@@ -230,7 +231,7 @@ public class AccountSetupBasics extends K9Activity
             mAccount.setTrashFolderName(getString(R.string.special_mailbox_name_trash));
             mAccount.setArchiveFolderName(getString(R.string.special_mailbox_name_archive));
             // Yahoo! has a special folder for Spam, called "Bulk Mail".
-            if (incomingUriTemplate.getHost().toLowerCase().endsWith(".yahoo.com")) {
+            if (incomingUriTemplate.getHost().toLowerCase(Locale.US).endsWith(".yahoo.com")) {
                 mAccount.setSpamFolderName("Bulk Mail");
             } else {
                 mAccount.setSpamFolderName(getString(R.string.special_mailbox_name_spam));
diff --git a/src/com/fsck/k9/fragment/MessageViewFragment.java b/src/com/fsck/k9/fragment/MessageViewFragment.java
index 711fcb93d..82dee9c42 100644
--- a/src/com/fsck/k9/fragment/MessageViewFragment.java
+++ b/src/com/fsck/k9/fragment/MessageViewFragment.java
@@ -2,6 +2,7 @@ package com.fsck.k9.fragment;
 
 import java.io.File;
 import java.util.Collections;
+import java.util.Locale;
 
 import android.app.Activity;
 import android.content.Context;
@@ -785,7 +786,7 @@ public class MessageViewFragment extends SherlockFragment implements OnClickList
     }
 
     private String getDialogTag(int dialogId) {
-        return String.format("dialog-%d", dialogId);
+        return String.format(Locale.US, "dialog-%d", dialogId);
     }
 
     public void zoom(KeyEvent event) {
diff --git a/src/com/fsck/k9/helper/Utility.java b/src/com/fsck/k9/helper/Utility.java
index f8392413a..5844c701f 100644
--- a/src/com/fsck/k9/helper/Utility.java
+++ b/src/com/fsck/k9/helper/Utility.java
@@ -26,6 +26,7 @@ import java.io.UnsupportedEncodingException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Locale;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -498,7 +499,7 @@ public class Utility {
             format = filename + "-%d";
         }
         for (int i = 2; i < Integer.MAX_VALUE; i++) {
-            file = new File(directory, String.format(format, i));
+            file = new File(directory, String.format(Locale.US, format, i));
             if (!file.exists()) {
                 return file;
             }
diff --git a/src/com/fsck/k9/mail/filter/FixedLengthInputStream.java b/src/com/fsck/k9/mail/filter/FixedLengthInputStream.java
index 48bf955a2..1125eef7a 100644
--- a/src/com/fsck/k9/mail/filter/FixedLengthInputStream.java
+++ b/src/com/fsck/k9/mail/filter/FixedLengthInputStream.java
@@ -3,6 +3,7 @@ package com.fsck.k9.mail.filter;
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Locale;
 
 /**
  * A filtering InputStream that stops allowing reads after the given length has been read. This
@@ -66,6 +67,6 @@ public class FixedLengthInputStream extends InputStream {
 
     @Override
     public String toString() {
-        return String.format("FixedLengthInputStream(in=%s, length=%d)", mIn.toString(), mLength);
+        return String.format(Locale.US, "FixedLengthInputStream(in=%s, length=%d)", mIn.toString(), mLength);
     }
 }
diff --git a/src/com/fsck/k9/mail/filter/PeekableInputStream.java b/src/com/fsck/k9/mail/filter/PeekableInputStream.java
index 0175bc4de..5a78c047c 100644
--- a/src/com/fsck/k9/mail/filter/PeekableInputStream.java
+++ b/src/com/fsck/k9/mail/filter/PeekableInputStream.java
@@ -3,6 +3,7 @@ package com.fsck.k9.mail.filter;
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Locale;
 
 /**
  * A filtering InputStream that allows single byte "peeks" without consuming the byte. The
@@ -59,7 +60,7 @@ public class PeekableInputStream extends InputStream {
 
     @Override
     public String toString() {
-        return String.format("PeekableInputStream(in=%s, peeked=%b, peekedByte=%d)",
+        return String.format(Locale.US, "PeekableInputStream(in=%s, peeked=%b, peekedByte=%d)",
                              mIn.toString(), mPeeked, mPeekedByte);
     }
 }
diff --git a/src/com/fsck/k9/mail/store/ImapStore.java b/src/com/fsck/k9/mail/store/ImapStore.java
index f9ecd9761..2363112eb 100644
--- a/src/com/fsck/k9/mail/store/ImapStore.java
+++ b/src/com/fsck/k9/mail/store/ImapStore.java
@@ -1284,7 +1284,7 @@ public class ImapStore extends Store {
                 int count = 0;
                 int start = 1;
 
-                List<ImapResponse> responses = executeSimpleCommand(String.format("SEARCH %d:* %s", start, criteria));
+                List<ImapResponse> responses = executeSimpleCommand(String.format(Locale.US, "SEARCH %d:* %s", start, criteria));
                 for (ImapResponse response : responses) {
                     if (ImapResponseParser.equalsIgnoreCase(response.get(0), "SEARCH")) {
                         count += response.size() - 1;
@@ -1347,7 +1347,7 @@ public class ImapStore extends Store {
         throws MessagingException {
             if (start < 1 || end < 1 || end < start) {
                 throw new MessagingException(
-                    String.format("Invalid message set %d %d",
+                    String.format(Locale.US, "Invalid message set %d %d",
                                   start, end));
             }
             final StringBuilder dateSearchString = new StringBuilder();
@@ -1361,7 +1361,7 @@ public class ImapStore extends Store {
 
             ImapSearcher searcher = new ImapSearcher() {
                 public List<ImapResponse> search() throws IOException, MessagingException {
-                    return executeSimpleCommand(String.format("UID SEARCH %d:%d%s%s", start, end, dateSearchString, includeDeleted ? "" : " NOT DELETED"));
+                    return executeSimpleCommand(String.format(Locale.US, "UID SEARCH %d:%d%s%s", start, end, dateSearchString, includeDeleted ? "" : " NOT DELETED"));
                 }
             };
             return search(searcher, listener).toArray(EMPTY_MESSAGE_ARRAY);
@@ -1505,7 +1505,7 @@ public class ImapStore extends Store {
             if (fp.contains(FetchProfile.Item.BODY_SANE)) {
                 // If the user wants to download unlimited-size messages, don't go only for the truncated body
                 if (mAccount.getMaximumAutoDownloadMessageSize() > 0) {
-                    fetchFields.add(String.format("BODY.PEEK[]<0.%d>", mAccount.getMaximumAutoDownloadMessageSize()));
+                    fetchFields.add(String.format(Locale.US, "BODY.PEEK[]<0.%d>", mAccount.getMaximumAutoDownloadMessageSize()));
                 } else {
                     fetchFields.add("BODY.PEEK[]");
                 }
@@ -1961,7 +1961,7 @@ public class ImapStore extends Store {
                 }
 
                 if (MimeUtility.getHeaderParameter(contentDisposition.toString(), "size") == null) {
-                    contentDisposition.append(String.format(";\r\n size=%d", size));
+                    contentDisposition.append(String.format(Locale.US, ";\r\n size=%d", size));
                 }
 
                 /*
@@ -2010,7 +2010,7 @@ public class ImapStore extends Store {
                 Map<String, String> uidMap = new HashMap<String, String>();
                 for (Message message : messages) {
                     mConnection.sendCommand(
-                        String.format("APPEND %s (%s) {%d}",
+                        String.format(Locale.US, "APPEND %s (%s) {%d}",
                                       encodeString(encodeFolderName(getPrefixedName())),
                                       combineFlags(message.getFlags()),
                                       message.calculateSize()), false);
@@ -2627,7 +2627,7 @@ public class ImapStore extends Store {
                 if (mSettings.getPathDelimeter() == null) {
                     try {
                         List<ImapResponse> nameResponses =
-                            executeSimpleCommand(String.format("LIST \"\" \"\""));
+                            executeSimpleCommand("LIST \"\" \"\"");
                         for (ImapResponse response : nameResponses) {
                             if (ImapResponseParser.equalsIgnoreCase(response.get(0), "LIST")) {
                                 mSettings.setPathDelimeter(response.getString(2));
diff --git a/src/com/fsck/k9/mail/store/LocalStore.java b/src/com/fsck/k9/mail/store/LocalStore.java
index c1389c4ba..e232c717d 100644
--- a/src/com/fsck/k9/mail/store/LocalStore.java
+++ b/src/com/fsck/k9/mail/store/LocalStore.java
@@ -223,7 +223,7 @@ public class LocalStore extends Store implements Serializable {
         }
 
         private void upgradeDatabase(final SQLiteDatabase db) {
-            Log.i(K9.LOG_TAG, String.format("Upgrading database from version %d to version %d",
+            Log.i(K9.LOG_TAG, String.format(Locale.US, "Upgrading database from version %d to version %d",
                                             db.getVersion(), DB_VERSION));
 
             AttachmentProvider.clear(mApplication);
@@ -1968,14 +1968,14 @@ public class LocalStore extends Store implements Serializable {
                                                                            type,
                                                                            name));
                                                 bp.setHeader(MimeHeader.HEADER_CONTENT_DISPOSITION,
-                                                             String.format("%s;\r\n filename=\"%s\";\r\n size=%d",
+                                                             String.format(Locale.US, "%s;\r\n filename=\"%s\";\r\n size=%d",
                                                                            contentDisposition,
                                                                            name, // TODO: Should use encoded word defined in RFC 2231.
                                                                            size));
                                             } else {
                                                 bp.setHeader(MimeHeader.HEADER_CONTENT_TYPE, type);
                                                 bp.setHeader(MimeHeader.HEADER_CONTENT_DISPOSITION,
-                                                        String.format("%s;\r\n size=%d",
+                                                        String.format(Locale.US, "%s;\r\n size=%d",
                                                                       contentDisposition,
                                                                       size));
                                             }
diff --git a/src/com/fsck/k9/mail/store/Pop3Store.java b/src/com/fsck/k9/mail/store/Pop3Store.java
index bdc0ef5bf..ee38e9a08 100644
--- a/src/com/fsck/k9/mail/store/Pop3Store.java
+++ b/src/com/fsck/k9/mail/store/Pop3Store.java
@@ -26,6 +26,7 @@ import java.util.LinkedList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 
@@ -510,7 +511,7 @@ public class Pop3Store extends Store {
         public Message[] getMessages(int start, int end, Date earliestDate, MessageRetrievalListener listener)
         throws MessagingException {
             if (start < 1 || end < 1 || end < start) {
-                throw new MessagingException(String.format("Invalid message set %d %d",
+                throw new MessagingException(String.format(Locale.US, "Invalid message set %d %d",
                                              start, end));
             }
             try {
@@ -791,7 +792,7 @@ public class Pop3Store extends Store {
                     if (listener != null) {
                         listener.messageStarted(pop3Message.getUid(), i, count);
                     }
-                    String response = executeSimpleCommand(String.format(LIST_COMMAND + " %d",
+                    String response = executeSimpleCommand(String.format(Locale.US, LIST_COMMAND + " %d",
                                                            mUidToMsgNumMap.get(pop3Message.getUid())));
                     String[] listParts = response.split(" ");
                     //int msgNum = Integer.parseInt(listParts[1]);
@@ -850,7 +851,7 @@ public class Pop3Store extends Store {
                               "Checking to see if the TOP command is supported nevertheless.");
                     }
 
-                    response = executeSimpleCommand(String.format(TOP_COMMAND + " %d %d",
+                    response = executeSimpleCommand(String.format(Locale.US, TOP_COMMAND + " %d %d",
                                                     mUidToMsgNumMap.get(message.getUid()), lines));
 
                     // TOP command is supported. Remember this for the next time.
@@ -872,7 +873,7 @@ public class Pop3Store extends Store {
             }
 
             if (response == null) {
-                executeSimpleCommand(String.format(RETR_COMMAND + " %d",
+                executeSimpleCommand(String.format(Locale.US, RETR_COMMAND + " %d",
                                      mUidToMsgNumMap.get(message.getUid())));
             }
 
diff --git a/src/com/fsck/k9/mail/store/WebDavStore.java b/src/com/fsck/k9/mail/store/WebDavStore.java
index 01b45655a..ea33e917f 100644
--- a/src/com/fsck/k9/mail/store/WebDavStore.java
+++ b/src/com/fsck/k9/mail/store/WebDavStore.java
@@ -1479,7 +1479,7 @@ public class WebDavStore extends Store {
             end = start + (end - prevStart);
 
             if (start < 0 || end < 0 || end < start) {
-                throw new MessagingException(String.format("Invalid message set %d %d", start, end));
+                throw new MessagingException(String.format(Locale.US, "Invalid message set %d %d", start, end));
             }
 
             if (start == 0 && end < 10) {
diff --git a/src/com/fsck/k9/preferences/TimePickerPreference.java b/src/com/fsck/k9/preferences/TimePickerPreference.java
index a2d6592b3..2675292e8 100644
--- a/src/com/fsck/k9/preferences/TimePickerPreference.java
+++ b/src/com/fsck/k9/preferences/TimePickerPreference.java
@@ -4,6 +4,8 @@
 
 package com.fsck.k9.preferences;
 
+import java.util.Locale;
+
 import android.content.Context;
 import android.preference.DialogPreference;
 import android.text.format.DateFormat;
@@ -94,8 +96,8 @@ public class TimePickerPreference extends DialogPreference implements
     @Override
     public void onTimeChanged(final TimePicker view, final int hour, final int minute) {
 
-        persistString(String.format("%02d:%02d", hour, minute));
-        callChangeListener(String.format("%02d:%02d", hour, minute));
+        persistString(String.format(Locale.US, "%02d:%02d", hour, minute));
+        callChangeListener(String.format(Locale.US, "%02d:%02d", hour, minute));
     }
 
     /**
@@ -106,8 +108,8 @@ public class TimePickerPreference extends DialogPreference implements
     protected void onDialogClosed(boolean positiveResult) {
 
         if (!positiveResult) {
-            persistString(String.format("%02d:%02d", originalHour, originalMinute));
-            callChangeListener(String.format("%02d:%02d", originalHour, originalMinute));
+            persistString(String.format(Locale.US, "%02d:%02d", originalHour, originalMinute));
+            callChangeListener(String.format(Locale.US, "%02d:%02d", originalHour, originalMinute));
         }
         super.onDialogClosed(positiveResult);
     }