Rules won't work that well. Beside that, you need a 24/7 moderator or moderator team to check every message posted here. And send out correctional matters to the writer of that topic.
You won't escape from the fact that people are tending to reply in their own language.
And beside all this, i haven't seen that much topics where someone started a topic a his/her own language and others reply in their own language.
And even so, Google Translate is a good tool to translate someone's reply.
And don't get me wrong here, but if your level of reading and understanding of English is at a poor level, it might be your own fault. Nowaday's everybody can read/write/speak English at a pretty good level due to English tv shows and movies and sitcoms etc. Maybe you should try to elevate your English language skills a bit more.
Or just do what i do when a topic goes all over the globe regarding the use of language: skip it and don't pay any attention to it.
