Well, I am now also taking German!
So far, it hasn’t intereferred with my Spanish, except I really want to say oder instead of “o” to mean or. Its hard. And, if I want to say something with the verb “Tu lo has hecho” its very easy to get “Du hast” in there somewhere!
But for the question about the swiss person, she doesn’t think in any language. Thought is independent of Language, but they are often tied up in them.
I bet you guys would love Stephen Pinker’s “The language Instinct” because it talks about things like this alot. His justification is that, sometimes, you can’t remember the word for a thought. Sometimes you can’t find the right word. Therefore if thought were only available in terms of language, then you would never think in terms of some term you don’t know.
As far as what Eats Crayons says, I’ll add to that a little, or modify it maybe.
I spoke english only for 20 years before I went to spain. After having been in Spain for 6 months, I was getting good in Spanish. Good enough to carry on a conversation with almost anyone about anything without missing a beat. So, when I came back, I started learning German.
Now, it has been MUCH easier to learn German than it was to learn Spanish. I really think learning a second 2nd language is much easier if you have gotten pretty good already in another language, because you have already learned to understand other languages. It seems like that skill works independently of what language it is. I haven’t translated German words into english. The true test of that is to ask yourself, or have someone ask you, “what is the word for window” and "what is the word for this: (picture of a window). If it takes you longer to answer the verbal question, then you are doing good. This shows that you are translating the english word into a visual image, and the visual image to das Fenster.