My project already has several locale folders. The .properties files in base locale en_US folder keep on changing as development goes and it is hard to keep the files in other locales in Sync.
Developed a software tool to manage locale files.
It is a standalone application that runs on Windows and requires .Net framework on the computer. It is a C# implementation.
The latest build can be downloaded from code.google.com/p/localemanager/. The C# source codes are also available there just in case if anybody is interested.
This software assumes that users follow the common practices for locale folders: use a separate folder for each locale. So, they normally look like:
locale-
|--- en_US - resource files in English
|--- de - resource files in German
|--- fr - resource files in French
It is very easy to use the locale manager. Just follow the screen captures:

The target locale file will be reorganized to use the same resource entries as in the base. Entries that exist in the base but are not in the target will be added to the target locale with a blank value. Entries that exist in the target but not in the base will be taken out and saved to a Mismatch_timestamp.log file located in the same folder as the EXE file.
Click Save Changes before closing the window, and changes made will be saved to the target file. The base file will not be touched.
The test input files I used only have 3 lines and the screenshot 3 doesn't look very impressive. It actually works pretty well with large sized files. When there are multiple files in each locale, the ComboBox can be used to select each file to work on them individually or work on all files at the same time.