They are Czech on paper, founders and most high level employees are Russian. Their main dev center used to be Sankt Petersburg, that's why Kotlin is named like that.
I don't know if their office in St Petersburg is still operating, since they claimed to stop sales and R&D activities in Russia: https://blog.jetbrains.com/blog/2022/03/11/jetbrains-stateme...
Is it still true? There were news of them selling their properties in St.Petersburg.