Errors in <ref> and <references> tags is currently rendered in the reader's assumed interface language, which might not be the ideal behavior. This task is to review the behavior, fix if necessary, and document on mw:Extension:Cite.
The weird part is that the errors show up in article text or in the References section, both of which are in the content language. Here's what it looks like:
English language reader sees:
https://en.wikipedia.org/wiki/2019%E2%80%9320_Villarreal_CF_season#References
Chinese language reader sees:
https://en.wikipedia.org/wiki/2019%E2%80%9320_Villarreal_CF_season?uselang=zh#References
Some arguments in favor of UI language errors:
- The error is most useful to editors. Most editors will have their interface language set correctly. They might have an easier time understanding complex technical points in their interface language.
Some arguments in favor of content language errors:
- The error messages don't include any UI element labels which might be translated. The only string literals are tag and attribute names, which are constant across UI and content language.
- Switching languages in article content is not done anywhere else (that I know of). CentralNotice has conflicting precendents.
- Cache split is slow for the first user from each language. Without a cache split, the content language rendering can be pre-cached for each new revision.
- Cache split is expensive for the cache, because content is duplicated.