**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:**
| [[ https://www.figma.com/file/UMHGYSxln03lGx2z7GxzP2/MUL---Termbox?node-id=340%3A5829&t=CoqKoUFLGxpqjPdE-0 | Link to flow in Figma ]] |
(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:**
```
Descriptions are disabled for this language (see help page).
```
* The link taget would be defined by a new interface message (the same message that we use for T316769 and T330193).
**Acceptance criteria:**
[] If an editor selects `mul` on Special:SetLabelDescriptionAliases then we deactivate the description field.