archwiki: show the main page's language button in the header
Enable $wgVectorLanguageInMainPageHeader
, so that the main page is consistent with other pages.
See https://www.mediawiki.org/wiki/Skin:Vector/2022#$wgVectorLanguageInMainPageHeader.
Compare: https://wiki.archlinux.org/?languageinmainpageheader=1 vs https://wiki.archlinux.org/?languageinmainpageheader=0
Discussed in https://wiki.archlinux.org/title/MediaWiki_talk:Common.css#Main_page_columns_and_Vector_2022_language_links.