I currently work on a Windows Phone 8.1 app for a client with an interesting approach to app localization. They have a Google Docs sheet with all the localization string for the app in all the supported languages and a tool that converts this localization strings in to per-language versioned JSON files. The generated JSON files are kept in a git repository. The Android, iOS and now my Windows Phone app should have the git repository added as a submodule. When a new version of the JSON files with the localization appear in the submodule, the app should use them.
Deciding the localization approach
In the old days of Windows Phone 7 and Windows Phone 8 I would simply write an utility that reads the newest version of the JSON localization files and generate a RESX file for each language. RESX is XML format that is simple to generate. If you had a localized string with a key say Game, you would put it everywhere when the localization of the word game was needed. Simple
In Windows Phone 8.1 (and 10) you should use RESW files for localization. This is the new x:Uid approach that, in my honest opinion, really sucks. It forces you to duplicate string if you use I string in multiple places, there is no design time support, you never know what UI element are localized and what you forgot to localize. Simply put, it is a mess. This approach is not usable at all with the string client provides.