My answer will be in English, just not to offend anyone in the group. Since the war broke out, many people have lost their friends/relatives, suffered a lot, and are upset.
I understand their sentiment. This group wants to become more pro-Ukrainian, more coherent & united, and the language is the first thing that might help with that.
If you are offended by this, please re-evaluate your position. I am speaking in Russian with my parents everyday on the phone, but I completely understand the intentions behind this Group's decision.