When requesting the page/summary endpoint for Chinese variant articles, it returns 400:
curl -i -H "Accept-Language: zh-hant" https://zh.wikipedia.org/api/rest_v1/page/summary/%E8%B4%9D%E6%8B%89%E5%85%8B%C2%B7%E5%A5%A5%E5%B7%B4%E9%A9%AC
When Accept-Language is not a variant, it works:
curl -i -H "Accept-Language: zh" https://zh.wikipedia.org/api/rest_v1/page/summary/%E8%B4%9D%E6%8B%89%E5%85%8B%C2%B7%E5%A5%A5%E5%B7%B4%E9%A9%AC
Debugging
Beginning almost immediately after the wmf.20 rollout we begin seeing many error responses to mobileapps from zhwiki /page/html with the message HTTPError: LanguageConversion is not enabled on this article..
https://logstash.wikimedia.org/goto/72e70761acc959574da0c9fd1e19e905
It seems that this patch fixes the issue, but didn't make the train https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/646794