This issue is still present, and the cause is that we only do prefix search. Injecting additional names such as Chinese (traditional) could work around the issue, if we cannot change from prefix search to word search due to performance concerns.

To be honesty, I can confirm on zhwiki that:

By using zh, zh-hans, zh-hant, zh-mo, and zh-my (suddenly want to say, why we can't drop it?), only zh-cn, zh-tw, zh-sg, and zh-hk shown:

When using one of these four, the one that you chose will be hidden (should be expected):

You can also just confirm it on any of WMF wikis, the results are per above.

On TWN, however, other than these four variants, I can also see Min Nan (Why? Why? Why?):

As per above, there's nothing to do under zh.* projects, hence removing that tag.

@Shizhao feel free to re-add it if there's something that T52270 not included.

Change 386158 had a related patch set uploaded (by Amire80; owner: Amire80):
[mediawiki/extensions/UniversalLanguageSelector@master] Add special language names to facilitate searching

https://gerrit.wikimedia.org/r/386158

Change 386158 merged by jenkins-bot:
[mediawiki/extensions/UniversalLanguageSelector@master] Add special language names to facilitate searching

https://gerrit.wikimedia.org/r/386158

The example still doesn't work.

The issue is that we have this much improved backend search but the frontend does not call the backend if it finds results locally. We should change that.

Change 393589 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[mediawiki/extensions/UniversalLanguageSelector@master] Improve ULS language search api

https://gerrit.wikimedia.org/r/393589

Upstream patch https://github.com/wikimedia/jquery.uls/pull/276

Change 393589 merged by jenkins-bot:
[mediawiki/extensions/UniversalLanguageSelector@master] Improve ULS language search api

https://gerrit.wikimedia.org/r/393589

@Nikerabbit - the patch was merged to master - does it mean that it's in translatewiki?

It seems that the issue is still present n translatewiki: entering the search term 'Chinese' will return the following:

The choices will lead to a page with the

Translations to this language in this group have been disabled. Reason:

This language code should remain unused. Localise in zh-hans or zh-hant please.

Entering zh-hans will correctly display

Note: In cx-testing, zh-hans and zh-hant are not recognized on ContentTranslation page. The ULS finds both.

Translatewiki.net was not updated. I updated it now.

Checked in translatewiki.net - both 中文（简体）(Simplified Chinese)
中文（繁體）(Traditional Chinese) are shown when 'Chinese' search term is entered.