I've read about people using a second language to learn a third language for two major reasons:

a) The process re-enforces L2
b) If L3 is more closely related to L2 than to L1, it could be easier (e.g. if you were a native German speaker who was fluent in Spanish, it might make more sense to learn Portuguese using Spanish than German due to the similarities)

I was wondering what your view (or anyone else's, for that matter) was on this idea. Do you think this would be a good idea, or would it be best to stick with the language that you know best as the "teacher" language?

Just to clarify the terms I'm using:
L1 = native language
L2 = non-native language in which the person is proficient/fluent (e.g. a language they learnt as an adult)
L3 = language which they are desire to study (and eventually raise to the level of the L2 language)

Yet another reason can be that there is more material available in one's L2. In case the L2 is English, one can study pretty much everything.

I've learned a whole lot of topics through English, including foreign languages. I may have used Swedish resources as well, but English has played a (major) part in just any activity I've spent any time on for the last 10-15 years.

I don't study through other langages for your two reasons, it's just that I'd be stupid not to use material in English (and to some extent German) for other languages, since I understand most of it.

That's a really good point that I hadn't thought of. If there's more useful content in L2 for your L3 than your L1, it makes sense to use L2 in that situation as well.

I think that if your L2 is at a good level, it really does not matter. I prefer to set my dictionary to English, but have used Chinese and Japanese texts to study Korean. It is probably a good thing to do, but I don't think it has a big impact. A more important factor may be, as Jeff says, what kind of learning materials are available.

Interesting question!

My second foreign language is German I guess, as it has overtaken Chinese some time ago (my first foreign language is Japanese). I actually attempted to set my lingq dictionaries to Japanese for my German study, but unfortunately not many helpful entries would come up at all. English is just much more useful online I suppose. Of course, German is also closely related to English, so I find it just makes more sense to study German using English as I am relating German words to similar English words in my head anyway, so why fight it?

That said, when I do study Chinese I tend to make use of Japanese online dictionaries. Again, this is because I find it easier to use my Japanese vocabulary to understand Chinese words than to use my English vocabulary. The fact that both languages use Chinese characters is a big factor here I suppose.

I guess it primarily concerns the language of the content available for you to study in for the language in question, and where that availability crosses over with your current knowledge. I imagined (possibly incorrectly) that the cross overs between languages would be more beneficial if you were studying in that language than if you were studying in a native, unrelated language. Although, I guess you would be able to pick up a lot of them anyway, as you would recognise them when you saw them. This may be a more important factor with languages in a different script to your native script (like danchan has said about using Japanese for Chinese).

The reason I ask, is that I've been thinking about what language to study when I'm comfortable with Japanese (which will be a long way off yet, but I like to think ahead). One of the languages I would like to study is Korean, and it seems there is a fairly large overlap between Korean and Japanese. It seems logical to me to use Japanese to study it, should my Japanese be at an appropriate level, but I haven't tried that idea for myself in the past.