i18n/temporary/en.json -> "wikimediamessages-desc": "Wikimedia specific temporary messages",
i18n/wikimedia/en.json -> "wikimediamessages-desc": "Wikimedia specific messages",
In extension.json there is "descriptionmsg": "wikimediamessages-desc",, so the first one seems incorrect. It's been there for 10+ years though.
There seems to be 73 usages of wikimediamessages-desc in i18n/temporary...