When my husband and I first met, I spoke Spanish pretty well and he spoke very little English. He was actually a student of mine at an English Institute in Santiage. Though, as soon as we were seriously dating, I could no longer really instruct him--as it often goes with couples. Our first 4-5 years together were "conducted" nearly all in Spanish.
(btw I never let him live down the fact that though he rarely corrected my Spanish, because he said it was perfect, he corrected my grammar once during a small tiff we had--which made me even more annoyed: He says "oh and after 'intentar' you don't use 'a'--it is hilarious now, but in the moment... not so much)
So when he came to the U.S. our modus operandi as far as communication started changing. As he learned more English, we spoke a more balanced amount of the two. Until we could finally argue, each in his/her native tongue--which must sound funny. But seriously, few things are more frustrating than getting angry in a language that isn't yours.
We spent many evenings during his first few months having conversations about vowel sounds and discriminating between words, like this:
He: Say "bicho" in English
He: Now say "bolsa"
He: They sound the same
Me: No they don't--look, bug/bag
He: ok, you said bolsa then bicho
Me: no I said bicho then bolsa
He: Ok, say them again... say one of them
He: you said "bolsa"
Me: no that was "bicho"
....... and on and on. It was endless.
Now, his English is nearly perfect. And he has a cute little accent to boot. But he still makes amusing little mistakes. One day I asked him what he thought of a new pick-up truck that was passing. He said he didn't like it because there was too much "blink blink" I said, you mean "bling, bling." Too funny. That is one of those things you hear but not necessarily see often, and the endings do sound really similar to a Spanish speaker.
He also says some expressions a little off. Like "Jeez, Louise" where Louise ryhmes with Jeez (loo-eez). Well for the longest time, he'd say "Jesus Louis" where Louis is pronounced (loo-iss) Doesn't ryhme at all.
(Which reminds me that when I was little I couldn't say "jee" because it was too close to Jesus--which was considered sacriligious to say like that. I couldn't say "gosh" either.)
He has also picked up on expressions here in Texas, that even I don't say. Once we had guests and I was in the bedroom with the baby. Hubs comes in and says "I think they're fixin to leave." The only logical response is: "did you just say fixin to". That just seems so Southern.
I love the language learning process, so it has been cool to see him go from beginning to fluent, picking up all the slang on the way.