Problem:
Users can select mul as a language on the Special:SetLabelDescriptionAliases page. If they fill out the description field and send the form, they will receive an error message, that says mul is not allowed for descriptions. We need to make this more intuitive.

Notes:

• The task T329626 does the same for Special:NewItem.

Mockup:

(based on Special:NewItem)

BDD:
If an editor selects mul on Special:SetLabelDescriptionAliases then we deactivate the description field.

GIVEN an editor has opened Special:SetLabelDescriptionAliases
WHEN the editor selects the language code mul
THEN the description field is disabled
AND the explanatory text (see "Copy") is shown below the disabled field (inline message)

Copy:

It is not possible to enter descriptions for multiple languages at once. Please provide a description after the Item is created.

Acceptance criteria:

• If an editor selects mul on Special:SetLabelDescriptionAliases then we deactivate the description field.