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.
Most of the people I knew who used to sit in St Petersburg back in the day when I worked at JetBrains are now somewhere more western. They have a pretty significant office in Munich, for instance.
Their offices in Moscow (Capital of Russia. Largest city), Saint Petersburg (Second largest city in Russia) and Novosibirsk (Third largest city in Russia) were suspended indefinitely in March 2022[1]