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.

/cc @archlinux/teams/wiki/admins