updating localization resources is never ending problem, there are few approaches that you may take but neither of them are bullet proof. Some of our clients use source control software in combination with WinMerge
so they can track changes made to resource files and decide if they should merge, delete or amend something. Another approach would be to use different resource files for your custom development. Or you can use DB resource provider and track changes via MonoX change scripts (a bit tedious approach if you are developer).
There's a thing with the localization that troubles me. Suppose I localize MonoX and then there's a new version out. Won't I need some diff tool to get my changes (calculated by the tool from the resources I had localized) and use some merge functionality to merge them later on into the newer build's resources? (or have some "translation memory" facility that can remember the pairs of original text and localized text and suggest them where appropriate when working on a future monox build's resources)
Google Translation manager is a great tool but I'm not sure that it can fit the MonoX purposes as there are too so much dynamic content that is generated by the modules. e.g. it will translate blog articles. That aside I'm not sure how easy would be to track the changes, your web app will depend on the Google service etc.