I've started learning Irish recently out of curiosity and because I can't speak any Celtic languages at all. I only plan to learn the basics because I can't imagine myself in a situation where I will actually need to speak it.
Latin is SO FUCKING BORINGI like overviews of languages and thus have done dozens of those, and routinely watch videos on it.
I learned Latin in school but that is kind of foundational to a lot of history, science and language in general so despite being a dead language I don't know if unpopular counts. Same for ancient Greek but did not go as far in that.
Learned the rudiments of Japanese because game translation but they are also a major world player so unpopular is a bit of a stretch.
Learned some Russian because why not but that still forms the basis of a lot of things and places I might go.
Tried my hand at some Finnish because I liked a lot of bands, films and such from there. For my money it was possibly the hardest to get to a reasonable level in which is almost the inverse of most I have ever done where most of the difficult things are in the initial hurdle where it gets hard again once you start wanting to do poetry, ancient texts (which might well be a different language for most intents and purposes), history/etymology of the language, clever double meanings and legalese type phrasing.
I had to learn grammar and text translation :'-) we only learned a tiny little bit of historyI had a great time in Latin. Learned more history than history lessons (not hard with awful teacher I had for that one), got to read all sorts of cool ancient texts that they make films and other adaptations of to this day (and I will probably have copies of on my bookshelf until I die where I might well cycle out the genero fantasy that makes up most of my fiction reading), made sense of a lot of language quirks and oddities that I had encountered in other things...