Not having any experience on Masamune, and thus no experience with the problem at hand, I'd really be careful with throwing the race card around. Not wanting to play with someone, regardless of the reason, isn't the same thing as actively engaging in crimes against/denying legal rights to someone based on reasons of race or ethnicity. They're saying they'd rather not play with you, not to go kill yourself for being _insert ethnicity here_.
I could easily say I don't want to play with non-English speaking or non-Spanish speaking players for reasons of ease of communication, rather than anything else. There was a post earlier in the thread that could have approximated a racist attack in the whole "Japanese people only" message in the party finder, but even that is suspect. Not saying JP players are any better or worse, but I think we can all admit to ourselves that the NA community DOES a reputation for having a high incidence of entitled assholes as well. Maybe Japanese players don't want to deal with people who are potentially "full of themselves" or whatever other reason and would rather stick with their own. Maybe there are other player communities in the French and German communities that feel the same way
That's just my initial perspective and possibly far from the reality, so take it with a grain of salt. That said, I really can't think of anything the OP can do outside of what several posters have already said. Report 'em, and while many think otherwise, SE DOES take valid(keyword) concerns seriously, or worse case scenario, bounce from the server. Sorry to hear about that. The game's community is toxic enough without this sort of thing happening.