I speak French fluently as a result of doing the French immersion school program growing up in Canada. I have used it a good deal since moving to Europe, in various aspects of my life. I adore the French language and I feel that this is truly one of the greatest gifts that my parents gave me, by enrolling me in the program, and the Canadian school system too!
I also studied Spanish from grade 9 through third year university, but I'd only call myself conversational there. I understand it really well and I can kind of get by, but it would need a lot of brushing up. I was pretty good after all those years of studying but I feel like I've wasted it a bit now.
I speak a tiny bit of Finnish as my mom is a Finn. She says she now regrets not speaking it to us more at home - she did, but perhaps not enough. I also spent summers in Finland as a kid but I was a shy child and it was too easy to let others translate. I think if I had continued to go for a few more years I would have made a breakthrough, but after my grandmother's death when I was 15 I have only been once since. I love the language but regret it as a missed opportunity, mainly.
It is so long since I started learning a new language. The last time was when I was 13, with Spanish... I think it would be much harder now. Life is busy and my brain is less resilient. Really useful languages would include Chinese and Arabic, I think. But I'm not sufficiently drawn to either to undergo the difficulty. Perhaps the most practical thing I could do would be to brush up my Spanish. I would also like to improve my little Finnish, but it's so not practical unless I want to go live there. I love German too and have a few words and phrases. And Welsh...but not so useful