"Language" characterization
Generally speaking, there exists no concisely-worded characterization of "language" notion. Usually, we see it as a combination of acoustic and written strictly-systematized symbols that are associated with corresponding concepts of the surrounding world. Therefore, "language command" is quite debatable notion as well. A person that is an expert at carrying on complicated negotiations has problems writing a simple letter or a note.
The notion of "language" is very doubtful due to the great number of dialects almost every modern language has. The core of the problem is that there is no clearly-cut line between "language" and "dialects" in modern linguistics. For example, diversity within the group of Turkic languages is rather insignificant whereas dialects of Chinese language are so diverse that a person needs a lot of time to master all of them.
Furthermore, determination of "language" and its difference from other "languages" is very often political issue. It is the story with above-mentioned Turkic ones that are very often said to be a group of the main Turkish language and its dialects. We have a resembling story with Serbian and Croatian languages that were firstly assembled and split afterwards. In Great Britain, it is the thing with Welsh, Scottish and Irish languages were either eliminated or assimilated with English.
In the issuance, the notion of "multilingualism" is very debatable in the same way being analyzed in all possible aspects in numerous scientific papers and on this site as well.
