Add empty locale extender to clear cache properly on enable/disable.

See https://github.com/flarum/core/pull/2020#issuecomment-616149583

extensions/markdown/extend.php

@@ -22,5 +22,7 @@ return [
             // Overwrite the default inline spoiler so that it is compatible
             // with more styling for children in an external stylesheet.
             $config->tags['ispoiler']->template = '<span class="spoiler" data-s9e-livepreview-ignore-attrs="class" onclick="removeAttribute(\'class\')"><xsl:apply-templates/></span>';
-        })
+        }),
+
+    new Extend\Locales(__DIR__.'/locale')
 ];