I want to understand this expression about "language", is it literally used or not?
Does the expression "a language" literally mean "language"? (for example, Korean, English etc?)
Yes. You need to look at the differences between countable and uncountable nouns.
A language = an/one example of a language (e.g. French is a language)
Language = an abstract idea or concept of all those things that can be described as "language."