I have started to code a small extension to override the language codes for the entries in the sidebar, ie to solve the "no" language code problem. It is only tracked by a single task T209089, but it would probably be cleaner to be able to refer to a project entry.
|Declined||jeblad||T209089 LangCodeOverride: Extension to override language codes in the sidebar|
|Declined||None||T209231 Phabricator project for an extension LangCodeOverride|
https://www.mediawiki.org/wiki/Extension:LangCodeOverride states that the code is located at https://github.com/jeblad/LangCodeOverride/ and that page already offers an "Issues" tab for tracking related tasks.
I strongly believe that tasks and feature requests shall not be reported in two separate places to avoid wasting users and developers duplicated time (and I less strongly believe that the code should preferably reside in Wikimedia Gerrit).