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, German, or Estonian, all three of which are because I frequently listen to lots of music that are in those languages
Haven’t picked back up consistently learning Japanese in like 8 years, but it’s a joy to feel the progress of learning such a vastly different language. I know enough Spanish that it’d be a waste to spend this gift on it.
I guess the most tactical option for me that’s still interesting would be Arabic, because while I love the idea of knowing it, I don’t think I’ll ever give myself the time to learn it.