hello,
i had similar issue for serbian language, since serbian lang can use 2 scripts: cyrillic or latin.
here is what worked for me: i used “SR” and “sr”. in your case, just use “zh” and “ZH” ??
also, you can use zh and xy in admin panel, and after that you can try to find xy in database and change it to zh (i have not tested that, but it might work as well)
regards,
mladen