- Currently, it is possible to specify language of Wikimedia maps by using syntax like https://maps.wikimedia.org/?lang=LANGCODE, for example https://maps.wikimedia.org/?lang=zh
- However, when the langcode is something like zh-HK with variants specified, the system cannot automatically identify fallback and fail to display zh-TW/zh-Hant/zh when zh-HK is not available.
- See also https://meta.wikimedia.org/wiki/Wikidata/Notes/Language_fallback https://zh.wikipedia.org/wiki/File:MediaWiki_fallback_chains.svg
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).
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.