I think until one learns a second language, you can't really fully appreciate some aspects of how the brain works. For instance, I have often heard people ask bi-lingual people what language is it that they think in, or dream in. It would seem that this could have a straight forward answer, but in reality it doesn't. One can see a table and think of it as a table, or as "桌子" or as "la table" or as "テーブル"- and all will mean the same idea of a flat surface with four stems for legs on it in the persons mind-any of which can be substituted depending upon the present context one if thinking of it. Even if one learns that language as an adult, there isn't necessarily one default language one must view all things. I think that might come as a surprise to some.
I do however find it easier to quickly reference some things in one language, while others things in another. I think this becomes just habit more than a set condition.
The most interesting thing about learning a new language to me, is that the integration of the new language happens almost as a subliminal transmission after a while, without one even really being aware that they have learned it, but the knowledge is just suddenly there somehow.