Multi-line literals in swagger editor?

It's the indentation. It should be like:

  post:
    summary: Translate one or more identifiers
    description: |
      Translate one or more identifiers for one entity into the
      identifiers of another entity. Translate one or more
      identifiers for one entity into the identifiers of another entity.

I believe the problem is the way you started the text on your description block. It must be indented one level to the right of description: Here's an example of something that works for me:

/{user-id}:
get:
  summary: Facebook User
  description: |
    Displays all information about a Facebook user, depending on access privileges.  Displays all information about a Facebook user, depending on access privileges.
  parameters:
    - name: user-id
      in: path
      description: The Facebook user ID
      required: true
      type: string

In my actual code, the description is three lines long.