I grew up in a fully bilingual home, in northwest Mexico. Then I was a foreign exchange student in Virginia, so my accent picked up a hint of southern twang.

So English is my second language, give me a minute of talking in it, and I'll start thinking in it, without the need of translating it in my mind. People tell me they can't place the accent... somewhere from Scandinavia, perhaps? French, maybe?
Someone once asked me if I grew up an army brat in Japan, and picked up some of the accent that develops in the communities around those bases, which is a fascinating concept.

Another friend, who's seen me go off in both Spanish and English, switching back and forth between people, told me that I have two personalities, and they're tied to the language, and they are like musical notes that go together.

EDIT: Now that I remember, what my friend said was "it's like you have two souls".

I think it was fortunate to have been enveloped by two languages so early on. Learning academically is important, but it'll only get you so far, the rest of the way requires that you be exposed to it, both passively and actively.

When I grew up, local cable had only stations from San Diego, if you wanted to watch cartoons, it would have to be in English.
You want motivation? I'll show you motivation: You learn English, and your reward is Bugs Bunny and Saturday morning cartoons. Every single one of my friends growing up spoke English and Spanish.

Less than a decade later, Mexican cable and satellite came into town. The English-speaking channels from San Diego got drowned out among a tsunami of new choices... and a huge motivation to learn English disappeared virtually overnight.

It looks to me like the type of environmental random experiment that pops up, like a Darwinian mechanism. Like growing a sharper beak to crack the nut. Then just as quickly as it popped up, it disappeared again, although an echo was left.