English is my first language, but I went to a Spanish bilingual immersion school from K-8, and then kept studying it in high school, so I'd say I'm pretty fluent in that. The switch from immersion to high school classes set me back and fucked me over a bit, because suddenly I had to question my grammar. When I've traveled abroad or used Spanish elsewhere though, I've found people really don't care if you use the wrong tense occasionally, especially if you're surrounded by asshole Americans who can't even be bothered to try. So in those situations, I've gotten better at switched back to my middle school Spanish. Of course, because I learned it immersion style, with a bunch of immigrants and second-generation Americans, I speak a very Mexican Spanish with a lot of street slang and more modern grammar than many of my high school teachers liked.
Being in an immersion environtment so long, with friends who all also spoke Spanish meant that I got used to being able to switch between the languages at will. I still have to watch myself with my high school friends because not all of them know enough Spanish for me to throw a word or two into the middle of a sentence.
I think in a mix though. Spanish can make some things a lot shorter, and other things a lot longer (for example, no possessive 's, which is a real pain). I also have picked up a few Mandarin words, some Italian and then just a few random other things from friends and other places that get mixed into my thoughts and speech. There's also a lot of teen slang I use, and some of it is pretty specific to my town, and even my high school. It's always kind of weird when I use a phrase with someone who lives elsewhere and they have no idea what I mean. Cool though too.