I think there may also be an issue with latency as well as language. I know that players in Japan have a much easier time dodging with their 9ms pings than those of us in Australia with our 120ms ones. While I imagine it's mostly that they don't want to have to deal with the language barrier, it could also be that they're sick of watching SEA players eat aoe. Either way, it's their prerogative to ask for members to be "jp only".

I also don't think that selecting all the languages on duty finder is a problem. I ran a few instances today with only EN selected and I got grouped with JP players (or players speaking Japanese, anyway) which didn't bother me at all. As long as you don't need any explanations it shouldn't really matter what language you speak.