"Language knowledge" is a compound noun. The noun "language" can be used as an adjective before many other nouns e.g. language ability, language lessons.

I see a article on italki I always write" language knowledge " The whole sentence is " If you already possess some knowledge of a language, then you can ask your italki teacher and online assessments to help you figure out your current language level." I

Could you give more context or the whole sentence? They could both be right.