This is an interesting thread for me.
I was brought up being exposed to three languages, none of which is The Queen's English as she is spoke in England, but rather Hibernian English which is not the same thing at all.. My Dh IS English and he sometimes feels like he's in a foreign country here in Ireland - well, technically he is...
My DD has just started school and spends a large portion of the day learning through Irish, and she also learns French, but not at total immersion level.
We speak Dora quite well also, and also a little bit of Sanskrit,(root words) as this is on the Irish course for the advanced leaving certificate level - my Irish teacher in school was a native Irish speaker and had studied Linguistics, so he wasn't actually a Irish language teacher per se.
My DH has a few words of Norwegian, I have a smattering of Dutch, Spanish, Arabic and Portuguese which we like to throw into the mix, and we babble a lot in made up languages, where intonation is the key.
We used sign language with DD from an early age as well, (and there were some raised eyebrows at that!)(BTW, we had no terrible two tantrums, as DD could 'tell' us what she wanted and how she was doing.)
We also sing a lot together in all the languages we can and recite poetry too, to get the feel of rhyming and rhythm. Dd is very good at doggerel as a result I think!
Now, Dd and I can talk about DH in Irish , her vocab is pretty strong in French, and we sing a lot of songs in French - her accent is very good - partly because I had a teacher who was not afraid of giving it wellie in class and spitting all over the front row and I follow suit.
Wherever I am I dream in the language of that country. When I was in total immersion Irish college during my school summers I would dream in Irish, when in France, I do as the french do etc..
I consider the brain rather plastic, (mine maybe more than a little porous also!) and think that the more languages the merrier. There are many people in the world, many songs, and it's lovely to be able to think, speak and sing in another language - the brain works in a different way and allows for greater lyrical flux.
When in Nursery DD picked up a bit of Chinese from one of the little boys there - she just flicked a switch in her brain to talk with him - like a change from major to minor key in music. I thought this was cool, as I didn't pick up a word of it when I worked over in that part of the world - I just couldn't get the ng sound or the intonation. The shame
Oh look, what fun, I've just noticed that there is a new emoticon - hurray
Humm.. Jammy Dodger or the Eye of Sauron eh?