I'll pick up Mandarin sooner or later. I have a lot of chinese-speaking friends in Singapore still and want to get into the culture more. It's just really intimidating to start with.
I did learn Japanese for a while but lost my motivation when it came time to learn Kanji. Can still read hiragana. I can read and write Hindi and speak it a little but lack vocabulary and confidence for an actual conversation.
Fluently speak only German and English. I used to be conversational in French and Spanish but decades of neglecting that leaves me with little more than the ability to recognize words.
I found Latin to be very simple to pick up, but it’s hard to be conversational now since it’s a dead language and there’s no one to speak it with. My vote would likely be Latin though, or possibly Greek
At any rate, learning languages is super fun and empowering. I can only recommend it to anyone. Even if you never get to a fluent level. You'll learn a lot more about your own language too, when understanding another's structure