The purposes that locales serve are grouped into categories, so
that a user or a program can choose the locale for each category
independently. Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as the first argument to setlocale.
The contents of the environment variable (or the string in the second
argument to setlocale) has to be a valid locale name.
See Locale Names.
LC_COLLATE ¶This category applies to collation of strings (functions strcoll
and strxfrm); see Collation Functions.
LC_CTYPE ¶This category applies to classification and conversion of characters, and to multibyte and wide characters; see Character Handling, and Character Set Handling.
LC_MONETARY ¶This category applies to formatting monetary values; see Generic Numeric Formatting Parameters.
LC_NUMERIC ¶This category applies to formatting numeric values that are not monetary; see Generic Numeric Formatting Parameters.
LC_TIME ¶This category applies to formatting date and time values; see Formatting Calendar Time.
LC_MESSAGES ¶This category applies to selecting the language used in the user interface for message translation (see The Uniforum approach to Message Translation; see X/Open Message Catalog Handling) and contains regular expressions for affirmative and negative responses.
LC_ALL ¶This is not a category; it is only a macro that you can use
with setlocale to set a single locale for all purposes. Setting
this environment variable overwrites all selections by the other
LC_* variables or LANG.
LANG ¶If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.
When developing the message translation functions it was felt that the
functionality provided by the variables above is not sufficient. For
example, it should be possible to specify more than one locale name.
Take a Swedish user who better speaks German than English, and a program
whose messages are output in English by default. It should be possible
to specify that the first choice of language is Swedish, the second
German, and if this also fails to use English. This is
possible with the variable LANGUAGE. For further description of
this GNU extension see User influence on gettext.