What a delightful, and in my limited experience, true idea. Having spent stretches of my life thinking mostly in english, then mostly in french, and now mostly in japanese and english, there is a noticeable influence on the basic kind and character of thoughts I have in each of the three. It isn't huge, but each has its own thought coloration and has concepts that are more easily expressed than the others.

Most fun for me now is talking with someone else that speaks all three, and watching the conversation just naturally blend them. Sometimes the blending is black/white (several sentences in one, then a statement in another state-changes the conversation into another language) and other times it ends up a mish-mash of two.