Library books are arranged by the Dewey Decimal classification scheme whereby each subject is represented by a number. You will find books on the same subject will have the same number. The shelfmark is preceded by the letter D and will be followed by 3 letters, which are usually the first 3 letters of the author's name.
Useful languages related subjects shelfmarks:
D 440 French language
D 840 French literature
D 944 French history
D 450 Italian language
D 850 Italian literature
D 945 Italian history
D 460 Spanish language
D 860 Spanish literature
D 946 Spanish history
D 791.43 Film studies