I had to give my opinion, which is that this idea sucks. What's wrong with people talking Dutch, Italian, Polish, French...? I don't care. And even if this rule will get there, I don't think much people will care about that rule.
I think that people should first of all have respect for the others. Yes you can speak whatever language you want however if some members (including myself) ask you to stop just stop. Same goes for only 2 people speaking in public chat (regardless of the language used) instead of just using the private chat.
Conversely, if more people are having a conversation in English please don't hijack it with another language. As well don't use the lack of English knowledge as an excuse, this website is meant, among others, to help you learn and improve your English.
To clarify, speaking your own language doesn't not entitle you to speak like you're on the street or at home, I will personally not tolerate foul language regardless of language or the people you talk to (e.g. your close friends).
As well, spamming or advertising is and will not be allowed.
When you enter the chatroom, a notice states that "English is preferred although not absolutely required." It's just a respectful thing to use English whenever possible. But we don't want to be nazis about it either. Bottom line is: use good judgement and be considerate.