Thanks for the updates, SpeechCenter. I didn't have time to test anything and I won't sooner than a week from now, but I really look forward to being able to use this tool.
As per the native language proofreading, there's a simple workaround to this, which I'm sure most people could have come up with by themselves, but I'll suggest all the same: creating a <Native Language> translation file and let a proofreader rework only the necessary lines. Then, to prevent the player from using the Default language, simply put something like this:
if (!IsTranslationAvailable()) //if the player is not using a translation it means he's using the Default Game language
Game.ChangeTranslation("<Native Language>"); //force him to use the <Native Language> translation instead.
It's a workaround, but really simple and not particularly inelegant, in my opinion. It works in AGS 3.1.2 SP1, but I'm sure equivalent functions exist also in other versions.
P.S. Though its name is misleading, IsTranslationAvailable() actually checks if a translation is being used or not, returning 1 or 0 respectively.