На. На дари, на фарси, на эсперанто. "Говорить по-(язык)" works when it's possible to form an adverbial, i.e. when the name of the language is an adjective with the suffix "ск": русский -> по-русски валлийский -> по-валлийски ирландский -> по-ирландски амхарский -> по-амхарски и т.д. Otherwise it's always "на", as far as I can tell. I'm not sure if there's a rule about it but I can't think of any exceptions.

Для меня естественно звучит только "на"