I don't know which is the language.
I don't know which the language is.
Note that this is, of course, a positive sentence and not a question.
Neither sentence is very natural-sounding.
I suspect what you mean is:
I don't know which language it is.
Without 'it' you are missing the object of the sentence that the verb 'is' relates to. Your mistake is in trying to make 'language' act as if it is the object, when in fact it is the thing the object of the sentence is being compared to.