Language missing from preview URL

The MR and the attached patch add a new service that helps in generating the draft, iframe and published URLs.

Note that there's an assumption that the entity is a node, which was already originally the case. The MR doesn't address that, so maybe something for a follow-up issue.

Also note that you may have to add the code below to you composer.json config when applying the patch through composer. Because the info yml file also has a change it conflicts with the d.o versioning.

"config":
  ...
  "preferred-install": {
    "drupal/decoupled_preview_iframe": "source",
    "*": "auto"
  },
...

Fixed an error when the default URL is used.

Thanks for the MR @klaasvw taking a look to this

I noticed the same issue, and the MR properly addressed it for me. Thanks! Marking RTBC.