LanguageCategory is an option for Cell which determines in what category of language the contents of the cell should be assumed to be for purposes of spell checking and hyphenation.
Possible settings for LanguageCategory are:
LanguageCategory is normally set to "NaturalLanguage" for text cells, and to "Mathematica" for input and output cells.
LanguageCategory is more often set at the level of styles than at the level of individual cells.
See The Mathematica Book: Section 2.10.11.
See also: $Language, Hyphenation, FormatType.