Some languages have codes that conflict with other standard tags needed by HTML, Mediawiki, or various Mediawiki extensions, and even for XML: the chosen tags should be scoped with a prefix or namespace but they are not. So this only works for the most common languages or languages encoded with a suffix (region code, or script coden or variant code after the hyphen).
And there's no support here to use (or chose) such prefix (or alternatively by limiting its scope with a container element with a more specific and predicatable name).
So this extension (unmaintained) already cannot be used in Wikimedia wikis with its hundreds supported languages and its many supported extensions (including those for translations and i18n).