On https://m.wikidata.org/wiki/Q42?uselang=de-formal, the new termbox (both server- and client-side rendering) emits lang=de-formal (MediaWiki language code) instead of lang=de-x-formal (BCP 47 standard language code). (See also T231475 for a related, general Wikibase issue.)
Info
@Lucas_Werkmeister_WMDE mentioned that termbox may get away with using the [[ https://www.wikidata.org/w/api.php?action=help&modules=query%2Blanguageinfo | languageinfo ]] API instead of [[ https://www.wikidata.org/w/api.php?action=help&modules=query%2Bwbcontentlanguages | wbcontentlanguages ]] as we are not in a Lexeme context (where wb specific languages really count), but adding bcp47 to wbcontentlanguages should be a manageable tasks as well - as was done for core during T220415: Add API module to get language information (which FTR happened after termbox implemented its language retrieval).
termbox pointers (implementing LanguageRepository): server, client
@Lucas_Werkmeister_WMDE Is this maybe somehow related to T312231: Investigation: Why does language fallback not work on Wikidata's mobile termbox?