I've been around a lot of people who were raised bilingual in English and an oriental language and most of them complained that it was too hard. They felt that at the high school and college level it's not possible to fully develop advanced vocabulary and usage in both languages. You can risk coming across as non-native or stupid in both languages. I think a lot of this stuff about bilingualism is biased toward people learning two related indo-european languages, which isn't that hard.
Perhaps it depends on the languages. I was raised in English; French was added starting at 5. Today, at 49, I am fluently and idiomatically bilingual in English, French, and various idiomatic variations on both common to the provinces in which I've lived (I can switch from stock-standard English to various idiomatic dialects and accents, as many of you can, e.g., formal English Vs the street or town English you spoke growing up, from those used here in Ontario to lilting dialects and accents of Nova Scotia; I can do the same with a few Quebec Joual dialects.)
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 don't think this has anything to do with learning languages. This kind of processing is also very common among mathematicians, physicists and programmers. More generally it is probably more common than you think especially among people whose default mode of thinking is not linguistic.
It happens to me a lot! And sometimes I can't find words in neither of my languages, and then turn to English (which is strange, since I learned English at a later point).
I was bilingual in English and German the first few years of my life, but the German gradually drifted away. I think the last time I had a German-language dream (that I recalled) was in my early teens. At this point I don't function well in German at all.
I think my English vocabulary is large and nuanced. Spelling and pronunciation can be a bit of a challenge at times, however. :)
