Unfortunately there is really nothing that SE, GMs or any authority can do about this.

The best we can do is...

1. Accept that we do not live in a perfect or necessarily even a fair world, and there are close-minded people in every society.

2. Know that they are also foolishly depriving themselves of the opportunity for friendship and the good experiences that can come out of mixed language parties.

3. Not give up.

4. And when you do get into Japanese parties (or any party for that matter):
- Be respectful, helpful, and courteous.
- Set a good example that will hopefully leave a lasting positive impression so they will want to play with you again.