This would be great if you wanted to talk to yourself without anyone understanding what you’re saying, for example, or if you didn’t want anybody to know what you were writing down. It might also be useful if you enjoy lots of foreign language media (see music, tv shows, movies) and want to experience the “original” rather than through dubs and subs.
Personally, I would go for either Russian or Estonian, both of which are because I frequently listen to lots of music that are in those languages, and they aren’t spoken by any of my friends.
Pictish.
It is a lost language from the British Isles and from one of the coolest populations in the area. They would paint patterns on their skin in blue then charge into battle naked. I reckon their insults and swearing would be absolutely raucous.
If you want some idea what it might have been like, it would probably be about as close to Welsh as Welsh is to Breton.
… which might not be all that helpful as a factoid, so here’s Wikipedia’s Swadesh list for the Celtic languages: https://en.wiktionary.org/wiki/Appendix:Celtic_Swadesh_lists
(In loosely related news, you know you might have been reading too much about linguistics when, while scanning the above, you recognise the Welsh word “benyw” as a cognate for the English word “queen”, (but with a meaning closer to Norwegian “kvinne”). That was a kick in the head for sure.)