for countries that have multiple languages, tolerance using other than the main language is a must imo. but if you are unified (fellow local members of the board), you must set other language rules that are prohibited in local posts.
Like I said in my previous post, it all depends on why you are posting in English.
Imagine that I mention your name and say something about you in the Croatian local. It can be either something positive or negative. Maybe even an accusation of some kind. It's normal that you would have an opinion about this and would like to share it. In that case, you have two choices. You can either make a normal post in English because you speak that language or you can make a silly and probably wrongly google-translated post in Croatian. Btw, I think such google-translated posts in the local boards aren't allowed. If it's not allowed, you are stuck with either saying nothing or writing in English.