This is a high-level tracking bug for a general issue -- part of the UX on a mediawiki site, especially in "off-the-main-article-namespace" areas like File: on commons or the workflow widgets used on Talk: -- is "user-generated content" but is more properly considered part of the "system interface" and should be rendered in the "user interface language" not the "content language".

This is a meta bug, so the particular issues this causes are best enumerated in the list of child tasks below. But broadly speaking:

• Doing this splits the parser cache by user interface language (or breaks rendering for certain users because the cache is not being properly split)
• Language variants are handled inconsistently: system interface messages are pre-rendered in a specific variant (eg sr-ec) but user generated content is in "all variants" (eg sr) and the system guesses which variant is used and tries to convert it to the user's preference.
• The Parser maintains a notion of "target language" but this can/should change when we enter a "user interface" region of the page. This is handled hackily by the {{int:}} parser function (see https://www.mediawiki.org/wiki/Help:Magic_words) but that only handles a single system message; when a UX component is generated by template we'd like to provide consistent facilities to user-generated content -- Special:MyLanguage and the various hooks provided by Translate help, but there's no unifying architecture here.
  
  We have a somewhat hacky way to split the parser cache to handle this,