Locale support refers to an application respecting cultural preferences regarding alphabets, sorting, number formatting, etc. PostgreSQL uses the standard ISO C and POSIX locale facilities provided by the server operating system. For additional information refer to the documentation of your system.
    Locale support is automatically initialized when a database
    cluster is created using initdb.
    initdb will initialize the database cluster
    with the locale setting of its execution environment by default,
    so if your system is already set to use the locale that you want
    in your database cluster then there is nothing else you need to
    do.  If you want to use a different locale (or you are not sure
    which locale your system is set to), you can instruct
    initdb exactly which locale to use by
    specifying the --locale option. For example:
initdb --locale=sv_SE
    This example for Unix systems sets the locale to Swedish
    (sv) as spoken
    in Sweden (SE).  Other possibilities might include
    en_US (U.S. English) and fr_CA (French
    Canadian).  If more than one character set can be used for a
    locale then the specifications can take the form
    language_territory.codeset.  For example,
    fr_BE.UTF-8 represents the French language (fr) as
    spoken in Belgium (BE), with a UTF-8 character set
    encoding.
   
    What locales are available on your
    system under what names depends on what was provided by the operating
    system vendor and what was installed.  On most Unix systems, the command
    locale -a will provide a list of available locales.
    Windows uses more verbose locale names, such as German_Germany
    or Swedish_Sweden.1252, but the principles are the same.
   
Occasionally it is useful to mix rules from several locales, e.g., use English collation rules but Spanish messages. To support that, a set of locale subcategories exist that control only certain aspects of the localization rules:
| LC_COLLATE | String sort order | 
| LC_CTYPE | Character classification (What is a letter? Its upper-case equivalent?) | 
| LC_MESSAGES | Language of messages | 
| LC_MONETARY | Formatting of currency amounts | 
| LC_NUMERIC | Formatting of numbers | 
| LC_TIME | Formatting of dates and times | 
    The category names translate into names of
    initdb options to override the locale choice
    for a specific category.  For instance, to set the locale to
    French Canadian, but use U.S. rules for formatting currency, use
    initdb --locale=fr_CA --lc-monetary=en_US.
   
    If you want the system to behave as if it had no locale support,
    use the special locale name C, or equivalently
    POSIX.
   
    Some locale categories must have their values
    fixed when the database is created.  You can use different settings
    for different databases, but once a database is created, you cannot
    change them for that database anymore. LC_COLLATE
    and LC_CTYPE are these categories.  They affect
    the sort order of indexes, so they must be kept fixed, or indexes on
    text columns would become corrupt.
    (But you can alleviate this restriction using collations, as discussed
    in Section 24.2.)
    The default values for these
    categories are determined when initdb is run, and
    those values are used when new databases are created, unless
    specified otherwise in the CREATE DATABASE command.
   
    The other locale categories can be changed whenever desired
    by setting the server configuration parameters
    that have the same name as the locale categories (see Section 20.11.2 for details).  The values
    that are chosen by initdb are actually only written
    into the configuration file postgresql.conf to
    serve as defaults when the server is started.  If you remove these
    assignments from postgresql.conf then the
    server will inherit the settings from its execution environment.
   
Note that the locale behavior of the server is determined by the environment variables seen by the server, not by the environment of any client. Therefore, be careful to configure the correct locale settings before starting the server. A consequence of this is that if client and server are set up in different locales, messages might appear in different languages depending on where they originated.
     When we speak of inheriting the locale from the execution
     environment, this means the following on most operating systems:
     For a given locale category, say the collation, the following
     environment variables are consulted in this order until one is
     found to be set: LC_ALL, LC_COLLATE
     (or the variable corresponding to the respective category),
     LANG.  If none of these environment variables are
     set then the locale defaults to C.
    
     Some message localization libraries also look at the environment
     variable LANGUAGE which overrides all other locale
     settings for the purpose of setting the language of messages.  If
     in doubt, please refer to the documentation of your operating
     system, in particular the documentation about
     gettext.
    
    To enable messages to be translated to the user's preferred language,
    NLS must have been selected at build time
    (configure --enable-nls).  All other locale support is
    built in automatically.
   
The locale settings influence the following SQL features:
       Sort order in queries using ORDER BY or the standard
       comparison operators on textual data
       
      
       Pattern matching operators (LIKE, SIMILAR TO,
       and POSIX-style regular expressions); locales affect both case
       insensitive matching and the classification of characters by
       character-class regular expressions
       
       
      
       The ability to use indexes with LIKE clauses
      
    The drawback of using locales other than C or
    POSIX in PostgreSQL is its performance
    impact. It slows character handling and prevents ordinary indexes
    from being used by LIKE. For this reason use locales
    only if you actually need them.
   
    As a workaround to allow PostgreSQL to use indexes
    with LIKE clauses under a non-C locale, several custom
    operator classes exist. These allow the creation of an index that
    performs a strict character-by-character comparison, ignoring
    locale comparison rules. Refer to Section 11.10
    for more information.  Another approach is to create indexes using
    the C collation, as discussed in
    Section 24.2.
   
    Locales can be selected in different scopes depending on requirements.
    The above overview showed how locales are specified using
    initdb to set the defaults for the entire cluster.  The
    following list shows where locales can be selected.  Each item provides
    the defaults for the subsequent items, and each lower item allows
    overriding the defaults on a finer granularity.
   
As explained above, the environment of the operating system provides the defaults for the locales of a newly initialized database cluster. In many cases, this is enough: If the operating system is configured for the desired language/territory, then PostgreSQL will by default also behave according to that locale.
      As shown above, command-line options for initdb
      specify the locale settings for a newly initialized database cluster.
      Use this if the operating system does not have the locale configuration
      you want for your database system.
     
      A locale can be selected separately for each database.  The SQL command
      CREATE DATABASE and its command-line equivalent
      createdb have options for that.  Use this for example
      if a database cluster houses databases for multiple tenants with
      different requirements.
     
Locale settings can be made for individual table columns. This uses an SQL object called collation and is explained in Section 24.2. Use this for example to sort data in different languages or customize the sort order of a particular table.
Finally, locales can be selected for an individual query. Again, this uses SQL collation objects. This could be used to change the sort order based on run-time choices or for ad-hoc experimentation.
    PostgreSQL supports multiple locale
    providers.  This specifies which library supplies the locale
    data.  One standard provider name is libc, which uses
    the locales provided by the operating system C library.  These are the
    locales used by most tools provided by the operating system.  Another
    provider is icu, which uses the external
    ICU library.  ICU locales can
    only be used if support for ICU was configured when PostgreSQL was built.
   
    The commands and tools that select the locale settings, as described
    above, each have an option to select the locale provider.  The examples
    shown earlier all use the libc provider, which is the
    default.  Here is an example to initialize a database cluster using the
    ICU provider:
initdb --locale-provider=icu --icu-locale=en
    See the description of the respective commands and programs for
    details.  Note that you can mix locale providers at different
    granularities, for example use libc by default for the
    cluster but have one database that uses the icu
    provider, and then have collation objects using either provider within
    those databases.
   
Which locale provider to use depends on individual requirements. For most basic uses, either provider will give adequate results. For the libc provider, it depends on what the operating system offers; some operating systems are better than others. For advanced uses, ICU offers more locale variants and customization options.
    If locale support doesn't work according to the explanation above,
    check that the locale support in your operating system is
    correctly configured.  To check what locales are installed on your
    system, you can use the command locale -a if
    your operating system provides it.
   
    Check that PostgreSQL is actually using the locale
    that you think it is.  The LC_COLLATE and LC_CTYPE
    settings are determined when a database is created, and cannot be
    changed except by creating a new database.  Other locale
    settings including LC_MESSAGES and LC_MONETARY
    are initially determined by the environment the server is started
    in, but can be changed on-the-fly.  You can check the active locale
    settings using the SHOW command.
   
    The directory src/test/locale in the source
    distribution contains a test suite for
    PostgreSQL's locale support.
   
Client applications that handle server-side errors by parsing the text of the error message will obviously have problems when the server's messages are in a different language. Authors of such applications are advised to make use of the error code scheme instead.
Maintaining catalogs of message translations requires the on-going efforts of many volunteers that want to see PostgreSQL speak their preferred language well. If messages in your language are currently not available or not fully translated, your assistance would be appreciated. If you want to help, refer to Chapter 57 or write to the developers' mailing list.