There is currently no use-case for descriptions in language code `mul`. Therefore, users should not be able use add `mul` for descriptions.
Disallow `mul` for descriptions on the API level (and therefore also via UI).
* 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.
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
 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).
No separate communication needed.