You will find below some guidelines for organizing an International Barcamp.
Spread the word if international barcamps are organized in a region close to your borders - or farther away.
The organizing language of the barcamp is English (newsletters, wiki, etc.), however, if the orga team is monolingual there is no reason to do the background organizing in English... Alternative version: You should advertise for the event using multilingual content, targeting the languages of the participants you want to invite. Of course, having also a version in English will probably reach out to the largest number of people.
The language of a session switches to English as soon as someone without the knowledge of the local language is present. (ask before you start if you are not sure.) If you are unable to hold a session in English, please note so at the session presentation and on the session sheet. Mark the sheet with DE, FR, NL, PL, etc.
You should have an introductory paragraph explaining the smallest common linguistic denominator rule: Me [sprechen|parler|parlare|pratar|...] no [German|French|Italian|Swedish|...], can I come? You are most invited to. As a rule of thumb, in our meetings we synchronise on the smallest common linguistic denominator. Which basically means, if there is a single person that does not speak the local language, we will switch to a language everybody understands (most of the time, we will switch to English).
Please also try to keep the networking language to English. Internationals will feel more welcome. Think about marking the nametags of the internationals with an (i*) or similar, so that it becomes obvious when they join a circle. We can be more polite this way.
Enjoy the multilingual atmosphere! Relax and have a drink, people will understand you better! :-) Have fun and learn new languages with foreigners in your home town (there are plenty of tandem language learning resources over the Internet)...