When I have a buyer who doesn't speak french or english, whatever the language, I use BABELFISH (I added a link to
http://babelfish.altavista.com in my Mozilla Firefox toolbar). Before to send the translated message, I verify it the best I can (italian, spanish or portugese are similar to french, so I can see if it makes sense and usually change some words to better fit).
For other languages, I ALWAYS add this message (translated) in capital letters at the beginning of my message : THE FOLLOWING MESSAGE HAS BEEN TRANSLATED USING A SOFTWARE. It's not perfect of course, but it's better than not be able to communicate with your buyer. Language has never been a problem on eBay, from my point of view.
The only real problem is when your buyer purchases an item without understanding the terms. Again, a short translated message can do the job. Being able to ask the buyer to modify his feedback is helping a lot for buyers who did not understand terms of sale or even item description.