With a couple of days in Paris, I am taken for a native. Same thing in New Orleans or Chicago (I have to be very careful about not drifting into local accents, I just sort of go there.)
So it is possible to be perfectly bilingual - but it requires early exposure to the second language and an ear (which can be developed - you have to stop listening to specifics, to particulars, and listen holistically, letting the language flow over you so that you understand the intent without understanding individual words - practice that for a while, and the words come).
I also speak passable remedial Spanish and have learned and forgotten a whole lot of Turkish, Arabic, German, and Greek.
After a while, your inner voice switches to non-linguistic (which I think might mean it returns to what it was before you had language, i.e., as a child). I used to think mostly in English, but nowadays I think in, well, viscera. I then have to find the words that most closely match the viscera. For simple things, that happens quickly. For complex things or subtle differences, it can take time, during which I sputter.
About 6 months ago I ask a friend's daughter whether this had happened to her yet (she was raised in French, added English much later, then some Spanish, and finally did intensive German prior to heading o'er the pond for her M.Eng).
Her eyes went wide and she shrieked "OUI! OUI! Exactement!" She turned to her mother: "M'man! Il comprend exactement qu'est ce qui m'arrive!"
(Yes, yes, exactly! Mom! He understands exactly what is happening to me!)
She'd been going through the same thing and didn't know it had ever happened to anyone else.
It's a truly strange thing, mind blowing - and mind expanding.
Sometimes, I can sort of disconnect from language. It frees my mind in a most powerful way. It's tricky to get there, though, and sometimes even harder to get back.
I think my English vocabulary is large and nuanced. Spelling and pronunciation can be a bit of a challenge at times, however. :)