I18N: Allow to short-circuit load_textdomain().

Introduces a new `pre_load_textdomain` filter, which is useful for plugins to develop and test alternative loading/caching strategies for translations. This brings consistency with the existing `pre_load_script_translations` filter for JavaScript translations.

Props ocean90, swissspidy.
Fixes #58035.

git-svn-id: https://develop.svn.wordpress.org/trunk@55928 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/l10n.php

@@ -717,6 +717,28 @@ function load_textdomain( $domain, $mofile, $locale = null ) {
 
 	$l10n_unloaded = (array) $l10n_unloaded;
 
+	/**
+	 * Filters whether to short-circuit loading .mo file.
+	 *
+	 * Returning a non-null value from the filter will effectively short-circuit
+	 * the loading, returning the passed value instead.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param bool|null   $loaded The result of loading a .mo file. Default null.
+	 * @param string      $domain Text domain. Unique identifier for retrieving translated strings.
+	 * @param string      $mofile Path to the MO file.
+	 * @param string|null $locale Locale.
+	 */
+	$loaded = apply_filters( 'pre_load_textdomain', null, $domain, $mofile, $locale );
+	if ( null !== $loaded ) {
+		if ( true === $loaded ) {
+			unset( $l10n_unloaded[ $domain ] );
+		}
+
+		return $loaded;
+	}
+
 	/**
 	 * Filters whether to override the .mo file loading.
 	 *