I do agree on your proposition to add the "_(language)" clarifier to language tags. Another way of naming it could be "_text", such as "english_text", "french_text", etc, to be more in line with tags like "english_commentary" and the like, but that's just a matter of preference - the idea itself is valid to remove ambiguity and prevent new and inexperienced users from making mistakes.
As for the nationality/place/etc thing, I think that's also a valid proposition - the gardening should have to be done manually but it'd be trivial to gather a list of nations via wikipedia and their respective adjectives and go through their corresponding tags here. We might want to also do stuff like making sure all *_flag, etc are added to qualifying images.
The ethnicity problem is a hard one. On one hand it's already hard enough to make sure that images with rare tags such as these are properly tagged, so standardizing them would make it easier for everyone, but on the other I have the same concern as keonas, where I can't see it ending in any other way than characters like Nitocris having every single picture tagged as egyptian_(ethnicity) just to make padding for the tag list, or everyone with remotely dark skin being tagged as african.