I18N: Apply the `locale` filter when returning early in `get_locale()`.

See [38976].
See #29783.

git-svn-id: https://develop.svn.wordpress.org/trunk@38978 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/l10n.php

@@ -53,10 +53,11 @@ function get_locale() {
 	// If $wpdb hasn't been initialised yet, we can only return what we have.
 	if ( ! $wpdb ) {
 		if ( ! $locale ) {
-			return 'en_US';
+			$locale = 'en_US';
 		}
 
-		return $locale;
+		/** This filter is documented in wp-includes/l10n.php */
+		return apply_filters( 'locale', $locale );
 	}
 
 	// If multisite, check options.