Steps to Reproduce:
Open https://m.wikidata.org/wiki/Q4115189?uselang=fkv as anonymous user
Some languages (e.g. fkv, dag, ...) are only shown as codes, not using their names.
Languages (e.g. fkv, dag, ...), which are WikibaseContentLanguages, i.e. added to the list of MediawikiContentLanguages, don't seem to be (consistently?) available in the client side equivalent wikibase.WikibaseContentLanguages.js - the client-side service definiton as used in termbox.
yields an empty result and termbox falls back to showing the language code only.
The server-side implementation (via wbcontentlanguages) successfully gets to the language names however. This can be witness by opening the example and quickly looking at the termbox before client-side rendering replaces it.
All valid content languages are shown using their name.
This was discovered when researching T261851.
The fallback to show the language code in case of unknown languages was implemented as part of T217770.