**Problem:**
There is currently no use-case for descriptions in language code `mul`.
**Solution:**
Do not allow `mul` descriptions on the API level (and therefore also via UI).
**Notes:**
* This could be achieved using a soft (see T289474) constraint (hardcoded, similar to T212869).
**Copy of error message:**
```
The language code “mul” (multiple languages) can only be used for Labels and Aliases, not for descriptions.
```
**BDD:**
If an editor tries to save a `mul` description then an error message is shown.
GIVEN an Item
WHEN an editor tries to enter a description in language code `mul`
THEN the edit is not accepted
AND and the error message (see copy above) is shown
**Acceptance criteria:**
[] If an editor tries to save a `mul` description then an error is shown.
[] The error message is translatable (and uses the correct language name for "multiple languages" as a parameter of the message).
**Community communication:**
No separate communication needed.
**Original:**
https://www.wikidata.org/wiki/Help_talk:Label#Drafting_of_guidelines_for_new_language_code_mul