I’m Finnish and besides the school English and Swedish I’ve been learning a tiny bit of Italian, Spanish, Latin, and enough Russian and Greek to be able to read names and other little things, slowly.
Japanese I can’t get any sort of grip on. So maybe something like that. A completely alien language. No! A properly alien language. Klingon. Or Sindarin. Or Lojban. I’m going to go with Lojban. That might not be totally alien, but it might be the language I’d like totally fluency in just to know what it’s like.