From personal experience, I spent about a month in my husband's home country last year, spending some time with the in-laws and some of his childhood friends, and I've been teaching myself his native language on and off for the last 2 years or so (with of course my husband's help). I'd say I'm at a conversational level, but not fluent by any stretch. I can talk with the in-laws in the language but if it's complex topics I kind of struggle, and I can't express myself as precisely as I would when speaking English. By the time the last day rolled around, I felt like I understood far more than I did when we first arrived there, even though my husband and I usually use a combination of both languages when we're at home. It's still maddening that my comprehension drops to like 5% of what's being said when two native speakers talk with each other (when I'm not part of the conversation) though. The hilarious thing was when we came back, for a period of about 3 or 4 days after we got back, I sounded quite foreign to my own ears when speaking English again, and everyone around me speaking English sounded like they had an extreme accent. I guess that was my brain suppressing the native language, as you said.