[mehrzad]

Not a website, and this may be obvious, but if you are doing research on Wikipedia on a topic from a non-English speaking country, sometimes that nation's language features a longer and more in depth article. I am currently learning German and have noticed this with regard to articles regarding universities in Germany.

[singularity2001]

this is a great recommendation also for scientific topics:

Often when I am really interested in something I read the article and four different languages, because most often each language offers different aspects and images.

also it's a great way to learn languages.