Six? Wow. The temptation would be to use the exact same font for all of them. Please resist. There's a typeface called Generation that might work great - see if you can track it down.

hhp

http://www.wallpaper.com/images/98_laurenz_brunner_am031207_f.jpg

The poster could use a dominant visual element that communicates its message non-verbally. It would be better than words in six different languages competing for attention.

And does it have to be a single poster, or can you split it up into two trilingual posters, three bilingual posters, or six monolingual posters?

Thanks for your suggestions. Using a visual element was my first idea, because six languages are really too much. Below is the image I've made.

The other one is to use only text in 'interesting' way. Maybe it can be done with only one font (using different styles, like Hrant suggests, for the different 'voices'), or can be done with totally different fonts, one for each language.

And to add another problem the arabic text can't be set in indesign. For that reason I think is better a font with the cyrillic than a font with the arabic language. What do you think of Gentium? (money is another problem)

I don't think my client want to split it in more than one poster.

Arabic can't be set in InDesign?

http://typophile.com/node/42608

This might actually be a dream project for Fedra: Don't forget it has 2 different serif variants, and the Sans has an Alt and a regular version – plus Condensed and Display variants; and all of those come in a ton of weights, so that should provide quite enough room for contrasts and «differences». You've got Arabic and Cyrillic covered (and Hindi)… and if all else fails, you can get a Phonetic alphabet too. :-)

Yes, I agree, Fedra would be a really good choice. (The problem for me is that after buying it I'd need a personal bailout.)