Discovered when working on https://phabricator.wikimedia.org/T356794. On 7.4.6+ (and earlier builds with Watchlist filters):

1. Set your device to English if it's not already, and add some Chinese variants (like Simplified or Traditional) to your Wikipedia languages
2. Open your Watchlist, tap to open Filter view
3. Observe the Wikipedias section

Expected result:
The localized language names should be displayed for all items in this section.

Actual result:
No Chinese variants are displaying their language names in this section.

I think this may be because the language variant codes don't seem to be transiting for these WKLanguages.