There is a fallback mechanism in place. Relevant fallbacks are described here. The problem seems to be that "zh" and its variants generally fall back to "zh-hans" that is little used on OSM (see taginfo). I assume these fallbacks should be rearranged, as long as Wikimedia maps localization relies on OSM tags.

Additionally the language picker is bugous, so that some languages that shouldn't be latinized get latinized (see T208927).

Is the system case-sensitive? Because the document use "zh-hans" and "zh-hant" while relevant keys in OSM are usually "zh-Hans" and "zh-Hant"

@C933103 and @Pikne Why this is a Language-strategy task? Comparing with task description, I'm not sure if we really need to "fix this problem" in all languages (or at least a batch of languages that ≥50) or not.

There were definitely >100 languages affected if language fallback didn't work in all languages, but if it's only for those that fallback to zh, and a few other languages that utilize script tag in Mediawiki project but utilize an alternative language tagging style in OSM projects, then the scope of the task could have been smaller than anticipated.

I have just realized something.
Are those codes supposed to be case-sensitive?
Because, if so, then that pull request wouldn't work
And that's also what I am seeing