Background: In MediaWiki various special pages may be display default or customized '*-summary' messages.

( Type 1 )

Common naming pattern for all these messages is '$specialPageName-summary'.
With page rendering these '$specialPageName-summary' messages placed into a div with class 'mw-specialpage-summary'.

These message may be empty by default, but if not, its content is displayed as header in the appropriate special page.

Examples:

• MediaWiki:Recentchanges-summary for Special:RecentChanges (WikiData)
  • currently combined with the MediaWiki:Recentchangestext
• MediaWiki:Search-summary for Special:Search (Meta)
• MediaWiki:Watchlist-summary/en and MediaWiki:Watchlist-summary/ru for Special:Watchlist (en) and Special:Watchlist (ru) (WikiData);
  • > but local message for "olo" (livvi-karelian) was not created, and nothing header is displayed.
• MediaWiki:Sp-contributions-footer, MediaWiki:Sp-contributions-footer-anon etc.

( Type 2 )

But in some cases, is still uses "legacy" messages, as 'MediaWiki:Recentchangestext' (see in Wikidata), that also shown in these same special page 'Special:RecentChanges'

This message is called by default way, as content language, a may be added in the '$wgForceUIMsgAsContentMsg' array for changing this behaviour (but this is not done anywhere).

In multilingual sites for this message currently realized solution with using content from translatable template, what displayed for users by depends from user interface language (by ugly way, through hacking msg 'MediaWiki:Lang', but this is worked).

Problems:

• For Type 1
  1. some summaries contains default text, included in main language file and may be translated in the translatewiki.net (as 'MediaWiki:Recentchanges-summary' )
  2. some other summaries is empty by default (as 'MediaWiki:Search-summary' or 'MediaWiki:Watchlist-summary'); translations for these messages will not be exported to the extension or MediaWiki core (see comment in the twn).
  3. these messages called as language specific message, with postfix '/ru', '/es' etc. based on the user-interfase' language
  4. therefore, for creating these messages locally, needs to create separate messages for each supported language (this is very actually for multilanguage portals like Commons and Wikidata). And if for your language this message was not created, you will not see anything - nothing default value not be substituted.
  5. Also no ways for using Extension:Translate to easy creation and maintainig these translations locally (without manual copying by sysops)
  6. See also code includes/SpecialPage.php

• For Type 2
  1. currently there is no common way for passing user interface language code to templates etc.; see also similar problem in T139463

Proposal:

• For Type 1
  1. Change MediaWiki code for using common way for calling '*-summary' messages by "Type 1", so that it will be as it is described in the mediawiki.org: by default, as content language); is needed, these messages may be included in the '$wgForceUIMsgAsContentMsg' (within instance settings)
  2. For 'UI messages', if translate is not present, should be substituted message for language defined by fallback chain, and if this also unexist - that default value from site content language.

• For Type 2
  1. implement the path for passing user interface language language code to templates etc. (this will be separate task)
  2. discuss and consider about keeping these 'legacy' messages as 'MediaWiki:Recentchangestext'