I had to put a similar stricture in WE PAW posting requirements. I only read and write English, so I cannot approve any post that is not in English. It's in our rules and guidelines.

I used to get a bit of flack for it when I first began to helm the group. The place was kind of a free-for-all and really without guided administration. The only thing I changed, was that and to enforce the rules the previous group owner had posted. It was rough going for a bit. The membership dropped from over 1,000 to around 250. But, that 250 appreciated the administration of the forum and since, everyone who has joined must agree to the rules first. Our membership has grown back to over 730.

Best off all, it's now rare than I or any of our members encounter unpleasantness. When that happens, I reign it in immediately and vigorously.