So I understand how Multilingual App Toolkit 4.0 works and it works great for my Xamarin.Android project. The default language is English, and with MAT we've translated it to Dutch and German.
Then comes the translator along and asks about how to make changes to the English texts, which is my source texts and the basis of all translations. The source texts (English) are in a file Resources\values\Strings.xml. Alter a string there, would invalidate all translations (Dutch and German).
Besides invalidation, I can't send my translator the normal .xlf file, because all source texts are in the Strings.xml file.
So my question is, how can my translator alter the source language texts in the most easy way (preferably with an .xlf file)?
The short answer is that you can't. The design of the Multilingual App Toolkit focused on providing easy support for translation into other languages. The concept of providing source content editing was not considered.
That said, as a workaround you could add another variant of English. For Android apps, the default is English Neutral (en), so adding en-US or en-UK would create the XLIFF file that your translator / proofer could edit using the same tooling (e.g.: Multilingual Editor) to provide source content proofing. If using the same translator for proofing and translating, they could apply the translation based on the update English string to the Dutch and German files.
When you receive the XLF files back ensure all the resource in the proofing English XLF (e.g.: en-US) are marked as need review, translated or final (basically anything except 'New'). The reason is that the Multilingual App Toolkit does not include untranslated resource in the generated target resource file. (The reason is for language fallback purposes in Windows apps).
Now you can import the proofed English file with the other XLF file and build. Copy the entire contents from the proofed English resource file(s) overtop the original source files (recommend Comparing first). When you next build, you will receive warnings that the source resources have changed after the translation, but you can just review and clear those warning in the editor, or clear them globally within the Visual Studio IDE.