**Problem:**
There is currently no use-case for adding descriptions in language code `mul`.
**Solution:**
Do not allow `mul` descriptions on API level (and therefore also via UI).
**Notes:**
* This could be achieved using a soft (see T289474) hardcoded constraint (similar to T212869).
**Mockups:**
**Error message:**
TODO
**Acceptance criteria:**
[] If an editor tries to save a `mul` description then the defined error message is shown.
**Open questions:**
Could we also find a more user friendly way of doing this? (e.g. deactivating the textbox)
**Community communication:**
Who we needs to keep in the loop and in what way:
Who this could be interesting for and in what way:
**Original:**
https://www.wikidata.org/wiki/Help_talk:Label#Drafting_of_guidelines_for_new_language_code_mul