After some various changes, current mode for Wikikidata 'Mainpage' is "single page for any languages" (with automated substitution translated version related with UI language).
Thus, many '`MediaWiki: Mainpage/*`' and redirects to '`Wikidata:Main_Page`' are no longer required.
**My proposal**: remove 'mainpage' from $wgForceUIMsgAsContentMsg for Wikidata, - for eliminating these redirects for all further languages. In my opinion, 'MediaWiki:Mainpage' should be called from any languages without language suffix.
This is very broad problem. As example, for 'bg' (Bulgarian language) links to mainpage from sidebar and from logo is leads to unexisted page 'Начална страница' (while the translation Wikidata:Main Page/Content/bg is completed).
See also [[ https://www.wikidata.org/w/index.php?title=Wikidata%3AContact_the_development_team&type=revision&diff=617497982&oldid=617490753 | related topic inwiki ]].