Why does the kotlin github repo README file not translate to other languages?

I think you need to expand a bit on that

Last year, a Korean developer submitted a PR for translating README, which was rejected

The issue is maintaining it. You don't want out of date translations, and if you start translating the README, where do you stop? I mean, it's pointless if not everything else is translated. But who will invest in such a thing? What if Finnish speaking folks want their translation too? What if they encounter a library they need, with an English README and doc? Yeah, learning English is simpler, we don't want yet another Babbel tower, at least I don't, and I'm not English native (French native).