Linguistics is the scientific study of language. The word 'language' is used in general sense not in particular, linguistics does not study of  individual lanhuage, it studies 'language' in general. According to Robins (1985), "linguistics is concerned with human language as a universal and recognizable part of the human behaviour and human faculties perhaps one of the more essential to human life as we know it, and one of the more far-reaching of human capabilities in relation to the whole span of mankind's achievement."
Linguistics does not emphasis on practical knowledge or mastery of a particular language. Linguistics may know only one language, or may know several, or may even study a language they do not know at all.