StringsMerge is a simple tool for merging Xcode strings files.
You simply load the older version and the newer version of the strings file on the app. Then it will generate a merged version, keeping the older keys and strings while appending the different keys and strings from the new version file.
The main feature of the app is that you are able to preview the generated strings file from within the app before exporting it to use in your project. It will tell you how many strings were merged, and warn you if it suspects any mistakes.