They're not really "for" a language. They are optimized subsets of the same font that only contain glyphs from one or more languages to minimize the file size.

They're not really "for" a language. They are optimized subsets of the same font that only contain glyphs from one or more languages to minimize the file size. If your website is written in English and an occasional accented character from other Western languages, there's no need to load a 50MB web font containing all the Tradntional Chinese characters.