2.9.22 International Messages
The standard set of messages for built-in Mathematica functions are written in American English. In some versions of Mathematica, messages are also available in other languages. In addition, if you set up messages yourself, you can give ones in other languages.
Languages in Mathematica are conventionally specified by strings. The languages are given in English, in order to avoid the possibility of needing special characters. Thus, for example, the French language is specified in Mathematica as "French".
Setting the language to use for messages.
This tells Mathematica to use French-language versions of messages.
In:= $Language = "French"
If your version of Mathematica has French-language messages, the message generated here will be in French.
In:= Sqrt[a, b, c]
Messages in different languages.
When built-in Mathematica functions generate messages, they look first for messages of the form s::t::Language, in the language specified by $Language. If they fail to find any such messages, then they use instead the form s::t without an explicit language specification.
The procedure used by built-in functions will also be followed by functions you define if you call Message with message names of the form s::t. If you give explicit languages in message names, however, only those languages will be used.