Of course you can post on multiple local boards. It’s one of the advantages of those that master more than one language, and one can even translate his own posts into multiple languages if he want to. What you can’t do is use translation tools to post in other languages (you therefore must master it on a decent level).

Regarding grammar, as I said before, you should have a decent command of those languages you post in; at least to make it understandable without having to infer its meaning. Now if the grammar is poor, your post may be reported by someone due to considering it of low value (if it turns out to be pretty incomprehensible). Then again, it may not be reported and live there happily ever after.

What you mentioned in the OP regarding the allegedly compromised account, well, that does happen. Sometimes accounts are hacked, sold or lent to another person, who in turn posts in a different language and/or style and/or sections. The different language is not a problem per-se, but rather more a possible thread from which to pull to uncover something.

This is now nearly a year old, but take a look at section 3 to see some forum members that have been merited on multiple local boards at some point (Analysis – The Multilingual gene).