Please document the feature created in T69223: Schema change for page content language and T153209: Actually enable Special:PageLanguage on all Translate wikis (et al).
I tried searching mw, but could not find anything (except for confusing similarly-named functions).

Suggested aspects:

• Create a section in the Extension:Translate help pages (user and developer)
• Disambiguate the mention of the magic word {{PAGELANGUAGE}} at https://www.mediawiki.org/wiki/Help:Magic_words#Technical_metadata (probably not needed)
• Disambiguate the older unrelated extension https://www.mediawiki.org/wiki/Extension:PageLanguage

The item in Tech/News that led me here, read:

Administrators and translation administrators can now use Special:PageLanguage on wikis with the Translate extension. This means you can say what language a page is in. The Translate extension will use that language as the source language when you translate. Previously this was always the wiki's default language. This was usually English. [1]