$Language is a list of strings which give the names of languages to use for messages.
All language names are conventionally given in English, and are capitalized, as in "French".
When a message with a name s::tag is requested either internally or through the Message function, Mathematica searches for messages with names s::tag:: corresponding to the entries "" in the list $Language. Only if it fails to find any of these messages will it use the message with the actual name s::tag.
See Section 2.9.22 and Section 2.14.1.
See also: MessageName, LanguageCategory.
New in Version 2.