MUC Administration Guidelines (for moderators)
The user should be given a polite, clear warning about their language, and asked to stop. In the case that they question the rules, direct them to the rules on the wiki page MUC Administration Guidelines. Unless they are obviously intentionally in the room to cause trouble, there is no reason to kick them before giving warning.
Please remember that people mostly come to the conference for help. They are new to Jabber, and should be welcomed, regardless of what language they speak. Direct them towards a room of their own language (see here MUC Administration Guidelines#English only for a list), so that they can be helped further. If 2 people are having an off-topic discussion, suggest that it be taken private.
The bot has some preset messages in various languages for guiding users in the right direction. Simply type !<country code>
If someone is sending you private messages, reply to them in the channel, and ask them to talk there. Point them here MUC Administration Guidelines#Don't send private messages if they question why.
Use your own judgement :)
It is unfortunate that many internet users are unaware of basic netiquette. Just politely let them know of their errors, and do not become one of them in your annoyance!