The message 'mainpage-title-loggedin' is responsible¹ for the "Welcome, $1!" message you see at the top of the main page in MobileFrontend. Since this message is directed at the user, I believe it should be in the user's language, and not the wiki's content language. This can be achieved by adding it to the $wgForceUIMsgAsContentMsg array in InitaliseSettings.php.
This is most important on multilingual wikis like Wikidata and Wikimedia Commons, but I believe this should be the setting for all wikis, since you can choose to change your interface language on any wiki.
Case in point: visit https://m.wikidata.org/wiki/Wikidata:Main_Page?uselang=nb . There, the entire page, including the interface and the main page proper, are in Norwegian Bokmål, except for the message that says "Welcome, $1".
¹ Via WikimediaMessages and the message 'wikimedia-mobile-mainpage-title-logged-in'.