I remember a few months ago, I guess they themselves hired spammers to shill in that thread, I remember because I reported a lot of posts from there to moderators.
Yep, they definitely hired shillers to bump their thread as I also reported some of them when I discovered that same group of accounts was working on 6 different ann threads.
Before that he was mostly speaking Chinese language and using machine translation to post in other local boards, than he switched to Russian language in 2019 and 2020.
It's possible that someone could speak both of this languages but I doubt that is the case here.
My conclusion: Account changed hands.
Looking at post history, I don't think that account changed hands. Its true that they most likely used translation tool to post in bunch of local boards (Indian, Chinese, Hebrew, Portuguese, French and few others ) but it was only done in relation to their project and they only made couple of posts in those languages, for example only 3 in Chinese. Huge majority of posts were written In Russian ( from 2017 to 2020) and in English ( from 2015 to 2021) so I don't see any drastic change of behavior.