"Česky" or "Čeština"?
Fragment of a discussion from User talk:Mormegil
I prefer the adverb for interwiki links because I would understand the interwiki link text as "in Czech", "in Upper Sorbian", "in Polish" etc. Behind the interwiki link is no page about a language but the article in the corresponding language. But I think the Polish form Polski is an adjective, the adverb should be po polsku.