Thank you Denis! Let me confirm this issue with better explanation in English.
If bbPress forum slugs are in cyrillic, following is happening while replying on some topic:
Let’s say I have set “тема” instead of “topic” and my new topic’s title is “Тема на русском” (“Topic in russian”). This topic’s url looks like http://www.myawsomenewforum.ru/тема/тема-на-русском/ and shows up just fine. Now let’s post a reply.
After hitting the “Reply” button an answer hash is being added just for a moment, following by redirect to the topic URL but already without “тема” in it. It gets stuck with a redirect loop on http://www.myawsomenewforum.ru//тема-на-русском/
