This is my problem. I studied French, and then Russian, then lived in Russia. I can understand spoken French well enough, but have to be very careful not to slip Russian words into my responses. Never the reverse. Numbers, for some reason, are especially difficult to keep totally in French.
I think this is true in the initial stages of learning a third language, but after a while they sort themselves out. My brain initially resisted Welsh, assuming it was some kind of bizarre German, even though I’d only had one semester of conversational German at the time. Now it’s hard for me to understand how one could mix the two.
I learned German first and Russian second a couple of years apart and never had any trouble confusing the two. When I tried learning French and Spanish concurrently, however, I encountered problems because the two are so closely related (“negative transfer”).
Over the last 20-odd years, I’ve used Russian much more than my German and have gotten very rusty in the latter; Russian words will now occasionally creep in when I try to converse in German, but I’m sure that if I spent some time in Germany I’d soon be fluent again.
Age has a lot to do with learning languages. My daughter (born in Moscow) was perfectly bilingual by the time she was 2 1/2; not a trace of an accent in either Russian or English. She started learning French at the age of six and, if she could go and spend a year in France, would speak it like a native.
As an aside, Czech and Polish (and I suppose Slovak) are much more difficult to learn than Russian, even though the three (four) are related. The vocabularies are much the same, but the grammar in the first two is completely arbitrary (like Latin) while Russian grammar, though quite complex, is very regular and predictable.
English and Portguguese I lived in Brazil long enough, and Sheckstress v1.0 was Brazilian.
And by fluent, I mean that not only do I converse, read, and write fluently, but I also use modern slang.
English and French, and some Spanish, Catala, and enough German to get around just fine as a tourist. Dad was in the service and we were stationed in Europe for about 12 years, I went to French school when I was 8.
I’m fluent in English and Afrikaans.
English, Hindi and Tamil - in order of fluency.
And the reverse order of cultural affiliation. Meaning, I was born into a family of Tamils in a Hindi speaking area. Learnt English pre-school and it quickly took over as the primary language.
I’m only fluent in English, I studied Spanish and can communicate the basics in that language. The weird thing is that my mom is fluent in English and French, and I studied French for a few years in middle school. I’ve pretty much lost the ability to speak it besides remembering vocabulary here and there. However, when trying to speak Spanish sometimes French comes out, and I don’t even consciously know the French word I said.
I guess it’s from hearing French sometimes while growing up (my mom pretty much only spoke English at home but sometimes French friends would come around and such) and the 7th-8th grade French classes. I would have continued with French and probably gotten near fluent with mom’s help, but I switched to a high school that only offered Spanish and German.
Living where I do, I speak and think in English, Mexican Spanish, and Spanglish. 
In high school I took three years of Russian. The only thing I retain from that is my handwriting makes no sense at all. “Why do you make a little H for n?”
A friend of mine is fluent (to the point of teaching) English, French, Japanese & German. She used to have an issue where she’d start talking in her sleep, having a conversation in one of those languages. The only way to get her to stop talking was to tell her to, in that language.
This somewhat limited her romantic opportunities.
I suppose Korean is my first language, although now my English is much better than my Korean. To me the big difference is when it comes to reading and writing. I wrote an MA dissertation in English, which was difficult enough, but I don’t think I could have done it half as well in Korean. My reading speed in Korean is a lot slower than my English reading speed.
But at certain times I’ll automatically speak Korean. The kinds of exclamations you blurt out when someone startles you, or you accidentally touch something hot - for some reason I always emote in Korean when something like that happens.
I’ve been in Korea for 5 consecutive years now (17 years in total) but my Korean is starting to get a wee bit rusty. I speak English at work and at home, so in the course of a regular day I have no cause to speak any Korean at all. Sometimes I’ll forget Korean words or make ludicrous spelling mistakes. I think a lot of foreigners here who teach low level English experience something similar - they start speaking very slowly and sometimes use broken English even when they’re talking to other native speakers.
English and Mandarin as a second language. I code switch. It’s weird that when I’m jamming along in Mandarin with people that understand English or English words, when I get to a word I don’t know how to say in Mandarin, then the English word just slots in. This happens a lot during technical discussions. Educated Chinese will know the English word and use it themselves in a mixed Chinglish fashion much of the time. Then when you deal with the elderly manager or uneducated Chinese, then I really have to struggle because I know they wont’ know the word.
Also, my mannerisms change depending if I’m speaking English or Chinese. It amuses colleagues.
After learning Mandarin, I find I’m “fluent” in Shanghaiese Cantonese and Japanese, but only in a very limited vocabulary. And that vocabulary get’s exhausted quickly. But if the conversation is 100% limited to the vocabulary I know, I’ll prattle on.
My kids all speak Mandarin as their first language and English as the second. They code switched from really young ages. Asian person then speak Mandarin, white person then speak English, elderly person then speak Shanghaiese.
I’m fluent in English, Strayan and Bogan.
Native English and speak Russian and Spanish.
Not many Swiss and Benelux dopers then. Or is that a misconception?
French and English
Most of my life is spent in French though!
Learned Spanish back in school but I haven’t had a chance to use it much.
I’ve been studying Latin as a spoken language for a few years now, and I’m certainly conversant, though fluent may be pushing it. I don’t do the kind of canonical code-switching you here about with, say, people on the bus speaking in Spanish suddenly turning a phrase in English without missing a beat, and then going on in Spanish. But I have always had this issue in speaking where once in a while I forget the word I need, for no particular reason. What’ll happen now is that when I blank on the English word, my brain retrieves the Latin equivalent. I don’t blurt this out automatically, but I kind of end up having to say it to clear it off the deck, if that makes any sense. I say it, and explain what it means until I remember the English word.
English, Russian, Hebrew - all fluent, although my Hebrew writing sucks. I can switch between Russian and English with no problem, but to switch to Hebrew fully takes a few days immersion.
For those people who have multiple languages at about same level of proficiency, here’s a question: when you do multiplication in your head (you know, the automatic rote multiplication table thing: “seven times eight is fifty six”), do you do it only in one language even if you’re currently in another language’s environment? I find that I do that in Russian, since that is the language in which I learned multiplication tables originally.
I think it depends on the language. When I am reading a language other than English and I see numerals, I have to make a mental note not to just read the numerals as English. It is easier for Italian than French, and easier for French than Welsh, because of how the numbers work:
99 = It. novantanove (ninety-nine)
99 = Fr. quatre-vingt-dix-neuf (four-twenty-ten-nine)
99 = W. pedwar ar bymtheg ar bedwar ugain (four-on-fifteen-on-four-twenty)
When I am just speaking a language, I can usually manage it just fine, though I often have to pause to think about large or complicated numbers, like 434 or 1942.
My only native language is English, though; it’s probably different for those with multiple native languages.