From a legal point of view... no it isn't legal, be aware that translations are a derivative right under the rights of the product you are translating. Releasing a game with a translation it is illegal by all means.
Now, having said that, a translation of a game if it doesn't contain anything else than the files translated should fall under the mod category, so releasing a translation isn't illegal, albeit it has no ownership rights unless it was consented by the owner of the game translated.
So basically:
-No consent from author = no ownership over the translation
-Consent = Ownership right of the translation exists
-releasing only the translation: No legal problems cause it falls under the "mod" category (it only works on games, other things such as books, articles... is illegal to do that).
-Releasing game with translation: It's illegal.
Having said that unless they can identify who did it there is no action for those acts thought