I18N: Add missing variable in string replacement.

Ensures the preferred file name for lookup has the correct extension.

Follow-up to [57337].
See #59656.
Built from https://develop.svn.wordpress.org/trunk@57338


git-svn-id: http://core.svn.wordpress.org/trunk@56844 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/l10n.php

@@ -821,7 +821,7 @@ function load_textdomain( $domain, $mofile, $locale = null ) {
 	if ( 'mo' !== $preferred_format ) {
 		array_unshift(
 			$translation_files,
-			substr_replace( $mofile, '.l10n.', - strlen( $preferred_format ) )
+			substr_replace( $mofile, ".l10n.$preferred_format", - strlen( $preferred_format ) )
 		);
 	}