Consider using zh-HK as a fallback language?
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Like what @Nemo bis: is doing now, I would like to phasedown number of languages which only translate from English, someone pointed that he oppose using zh-Hant for Cantonese because most of zh-Hant contributors are from Taiwan instead of Hong Kong, that's very true. But I believe if using zh-HK instead of zh-Hant, then we could simply clean up untranslated messages in order to continue working on East Asian languages' compatibility investigations. Any suggestions here are welcome.
@Nemo bis: Still this needs your comment.