Adding add-ons to the forum includes all kinds of risks. In this case I don't have enough confidence in the developer so I can't take the risk.
Yeah, I can understand that. However, we still have the problem with signal-to-noise ratio getting worse and worse. That's not critique, it's just a side-effect of us getting more and more members posting in language sub-forums, which most of the rest of us can't read.
I have no idea how receptive XenForo is for new ideas, but perhaps I could at least wish for you to post a wish on their site, for them to add some sort of user-configurable forum/thread filter into the base forum package to a future version?