Learning a new language is always powerful long term I think. I wouldn't learn a new language to add to your channel, but if you learn a new language, it could definitely help you in translating your captions for a specific language, and if you get a lot of comments in that language.
I get a lot of Spanish speakers, so I've considered learning Spanish so I can reply to some of my comments in a native tone instead of using Google Translate. Google Translate is probably full of errors in grammar
It's always nice to expand your knowledge with different language skills.