The site you're describing sounds very much like how Wikipedia works; you could probably draw a lot of inspiration of how they built it.
I would personally only show the games that are translated. I always find it very annoying to click on a link and get message like "Sorry, we're working on this, please come back later"
I'd say volunteers can either adapt the game to their own country/region, use the US version (which probably a lot harder for them than it is for US people), or not translate it at all.
No, I think would ask one or two other volunteers to compare the translation to the English version, and correct any mistakes.
I think I'd leave the form there and translate any comments via Google Translate to check if the comment is relevant for that page (i.e., make sure it's not spam).
No, if they have a game that is specific for their country (see question 2, but the other way around), I'd let them submit that.
That's probably a good idea, yes.
I don't understand the question. Can't you just make the whole site in UTF-8?
Beside translating the games, you probably need other texts translated as well, like a home page, contact page, stuff like that. You need to think about who should translate those.
Other than that I think you pretty much covered it.