For Android and iOS:
suppose I start with English as source for translation, I want to translate into French, Italian and Spanish. I already made a lot of translations for these languages, how can I upload them into the toolkit?
I can't say I have any idea, but I feel bad no one has responded to this point. The best I can do is this documentation here for creating and uploading new translation memories.
Unfortunately, since I'm not among the restricted users, I can't access the API documentation.
It seems like Google hasn't done much at all with this offering, but I wish you luck with it.