I've turned down sessions because the language barrier seemed like too much of a potential problem. I'd be laxer about it for a basic foot session but anything more complicated requires a certain level of communication. At a minimum we both need to be able to understand each other's limits.

I also tend to be very verbal and I don't enjoy myself as much if I feel they can't fully understand me. I feel like it handicaps my ability to give them a good experience.

Foreign accents can be cute as hell, though.