Why it is not respected? When someone visit this domain it redirects to english main page.
Topic on Project:Village Pump
Sidebar displays the main page correctly when the user language is set to pl, but the logo and the www.mediawiki.org domain does not. I guess the main page is not respecting the localization, which it makes sense, since default MediaWiki installation has localized main page titles which won't have any content on the vast majority MediaWiki installations.
However, changing this seems to be possible as described in Manual:$wgForceUIMsgAsContentMsg. You can report it to phabricator so this setting is enabled for mainpage.
Yes, this may be this variable. I found this for Meta and it works. Result I expect is if user set, for example PL, then if type only mediawiki.org then it show MediaWiki/pl as the main page instead MediaWiki.