l10n: Update wp_get_installed_translations() to support variants of a language.

* A variant of a language has its own locale, for example the locale of the formal variant of German is `de_DE_formal`.
* Update `remove_accents()` and some CSS rules to support `de_DE_formal`.
* Add tests for `get_bloginfo( 'language' )`.
* API changes will be deployed over the next few days.

see #28303.
Built from https://develop.svn.wordpress.org/trunk@33027


git-svn-id: http://core.svn.wordpress.org/trunk@32998 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/l10n.php

@@ -820,7 +820,7 @@ function wp_get_installed_translations( $type ) {
 		if ( substr( $file, -3 ) !== '.po' ) {
 			continue;
 		}
-		if ( ! preg_match( '/(?:(.+)-)?([A-Za-z_]{2,6}).po/', $file, $match ) ) {
+		if ( ! preg_match( '/(?:(.+)-)?([a-z]{2,3}(?:_[A-Z]{2})?(?:_[a-z0-9]+)?).po/', $file, $match ) ) {
 			continue;
 		}
 		if ( ! in_array( substr( $file, 0, -3 ) . '.mo', $files ) )  {