This is a really interesting topic
I think I'm in the minority on this thread in that I've not been brought up speaking any other language; I was born in the north, have no overseas family ties and have had to learn these things from scratch starting at quite a late age as far as languages go (in my teens). I don't travel abroad nearly as much as I'd like, either. So just to compare experiences...
My French is passable - I did the A-level at college, can read and write reasonably and just about understand bits of their TV and radio broadcasts, but I've no idea if I'd be able to hold a meaningful conversation in it; I suspect not. My current hare-brained scheme is learning German - never had the chance to do it at school or college but did an evening course at the Goethe-Institut in Manchester a couple of years ago (had to give it up in the end because my work at the time clashed, might see if I can pick it up again now things have changed!)
Anyway, people here always tell me I make it look easy, but I actually find it terribly difficult to learn new languages to any sort of meaningful level when I have to make do with broadcasts and internet pages etc. rather than conversation. I found that last time I left the country (to Germany for a couple of weeks last year) by the time I'd been there a week and a half I was actually able to talk to people without stuttering so much, though. Anyway, I'm rambling on like a right prat, but to sum up, I honestly think that the only way you can learn to be fluent in a language unless you're some kind of savant is immersing yourself in it for a lengthy period. I'm finding it an awful lot easier, though, to pick up German than I did French - and I do wonder whether it's because when learning French I had to get used to using a different grammatical scheme to English for the first time, now I'm on German it's
another different scheme so isn't quite as confusing.
With my French vocabulary, I've found that I can actually read some very basic Spanish - half the time, it's a similar word spelled differently! I reckon if I actually decided at some point to learn it, I'd be able to pick it up quite easily with a little effort.
(I should really work on making my English readable and not rambling like this!)
c