Currently, it appears as if editors can add `mul` descriptions in the termbox. But when they try, they receive an error message, that says `mul` is not allowed for descriptions. We need to make this more intuitive.
* In story writing, it was decided to split the original task into viewing (T329644) and editing (T338302).
* T332174 does the same thing for desktop
* The reason is that we only allow the language code `mul` for Labels and Aliases and not for descriptions (T313027).
* Remove descriptions for `mul` language code from the users' termbox workflow
* If a WM message already exists for "-" (disabled field) then use that.
| [[ https://www.figma.com/file/UMHGYSxln03lGx2z7GxzP2/Multiple-languages?node-id=340%3A7833&t=gYKEPhz7t1MGXkxv-0 | Link to Figma specs ]] |
 When editing an Item on desktop, the termbox shows a disabled edit field for descriptions for `mul` (see mockup).
 Release to test.wikidata.org