Personally, I think that it's disrespect to post in English language in your own local board unless posts are intended for international users. That's definitely okay if your country has two or more official languages and one of them in English, like in India and Ireland.
Btw no one can prohibit you from posting in English in local board, it's just disrespect that will probably upset a lot of local board members if you intentionally don't post in your own language and post in English.
Sometimes while posting in international boards, many members get into the habit of posting in English, as a result of which they cannot write good posts in the local language even if they want to. So they also want to post in English on local boards, which many local members find difficult to understand because English is not their main language. But I do not agree with the statement that posting in English on the local board will upset the local members and disrespect the local board.