Interesting question! Just some thoughts (not intended a a complete solution, it's more a set of questions):
A straightforward approach is having an internal tag ID and for each language a localized name.
If no localized name was created yet, you may need to fall back to the tag name in a 'primary' language - usually english - or the language the tag was created in.
Translation needs to be done by a user ho knows both languages, automatic translations are IMO to imprecise. So probably a user right (bound to rewards?) to rename tags.
Are all languages equal, or are tags only created in a "primary language" understood by most users, and translations added separately? (The latter looks less fair, but would probably make some things easier)
You need an ability to merge tags - e.g. when users independently created "discothèque" and "nightclub".
Do I see only the tags that are available in my language, or can I see tags available in other languages that don't have translations to my language? Can I search for tags in other languages?
Is the tag name included in a query string? Will my german query link work when I send it to a friend in the US?
How to resolve disputes regarding the tag meaning? Example: The closest translation to german is "Nachtclub" for night club, and "Diskothek" for "discothèque". But in german, a "Nachtclub" is quite different from a "Diskothek" (though there is some overlap).