object or database with ICU as the
     provider, the given locale name is transformed ("canonicalized") into a
     language tag if not already in that form. For instance,

<screen>
CREATE COLLATION mycollation3 (provider = icu, locale = 'en-US-u-kn-true');
NOTICE:  using standard form "en-US-u-kn" for locale "en-US-u-kn-true"
CREATE COLLATION mycollation4 (provider = icu, locale = 'de_DE.utf8');
NOTICE:  using standard form "de-DE" for locale "de_DE.utf8"
</screen>

     If you see this notice, ensure that the <symbol>provider</symbol> and
     <symbol>locale</symbol> are the expected result. For consistent results
     when using the ICU provider, specify the canonical <link
     linkend="icu-language-tag">language tag</link> instead of relying on the
     transformation.
    </para>

    <para>
     A locale with no language name, or the special language name
     <literal>root</literal>, is transformed to have the language
     <literal>und</literal> ("undefined").
    </para>

    <para>
     ICU can transform most libc locale names, as well as some other formats,
     into language tags for easier transition to ICU. If a libc locale name is
     used in ICU, it may not have precisely the same behavior as in libc.
    </para>

    <para>
     If there is a problem interpreting the locale name, or if the locale name
     represents a language or region that ICU does not recognize, you will see
     the following warning:

<screen>
CREATE COLLATION nonsense (provider = icu, locale = 'nonsense');
WARNING:  ICU locale "nonsense" has unknown language "nonsense"
HINT:  To disable ICU locale validation, set parameter icu_validation_level to DISABLED.
CREATE COLLATION
</screen>

     <xref linkend="guc-icu-validation-level"/> controls how the message is
     reported. Unless set to <literal>ERROR</literal>, the collation will
     still be created, but the behavior may not be what the user intended.
    </para>
   </sect3>

   <sect3 id="icu-language-tag">
    <title>Language Tag</title>

    <para>
     A language tag, defined in BCP 47, is a standardized identifier used to
     identify languages, regions, and other information about a locale.
    </para>

    <para>
     Basic language tags are simply
     <replaceable>language</replaceable><literal>-</literal><replaceable>region</replaceable>;
     or even just <replaceable>language</replaceable>. The
     <replaceable>language</replaceable> is a language code
     (e.g. <literal>fr</literal> for French), and
     <replaceable>region</replaceable> is a region code
     (e.g. <literal>CA</literal> for Canada). Examples:
     <literal>ja-JP</literal>, <literal>de</literal>, or
     <literal>fr-CA</literal>.
    </para>

    <para>
     Collation settings may be included in the language tag to customize
     collation behavior. ICU allows extensive customization, such as
     sensitivity (or insensitivity) to accents, case, and punctuation;
     treatment of digits within text; and many other options to satisfy a
     variety of uses.
    </para>

    <para>
     To include this additional collation information in a language tag,
     append <literal>-u</literal>, which indicates there are additional
     collation settings, followed by one or more
     <literal>-</literal><replaceable>key</replaceable><literal>-</literal><replaceable>value</replaceable>
     pairs. The <replaceable>key</replaceable> is the key for a <link
     linkend="icu-collation-settings">collation setting</link> and
     <replaceable>value</replaceable> is a valid value for that setting. For
     boolean settings, the <literal>-</literal><replaceable>key</replaceable>
     may be specified without a corresponding
     <literal>-</literal><replaceable>value</replaceable>, which implies a
     value of <literal>true</literal>.
    </para>

    <para>
     For example, the language tag <literal>en-US-u-kn-ks-level2</literal>
     means the locale with the English language in the US region, with
     collation