I firmly believe that just its name set TeXmacs back by a lot. It's a pretty great choice, was even more so a decade or two ago (when typst and the ilk weren't available). But every single time I've tried to introduce it to someone, it's always some variant of "Oh, I don't use Emacs, so a plugin for that isn't useful to me". It's a really unintuitive and unfortunate naming choice that I wish they'd at least changed at some point along the way.

Any idea about TeXmacs support for non-latin languages?

There is some support, but I do not know the details. You might try and ask in the TeXmacs forum, at http://forum.texmacs.cn/; a few of the developers read it and answer questions and they might have the information you are looking for.