When a field for the url translations is added to the page properties, let's say "name", then the editors can fill in values in all languages, including the default language. But the url translation module completely ignores the value for the default language, and only uses the page name. While this might make sense from the technical POV, this leads to confusion for the editors.

Workaround
Define a URL Translation tab which defines fields for each language. Do not internationalize the name field but instead create a dedicated tab for URL Translation which defines a field for each language you need to capture.

For example if I decorate the MTK2 basic page properties.
/my-module/decorations/mtk2/dialogs/pages/basic.yaml

form: 
  properties: 
    name_de: 
      label: German
      $type: textField
      i18n: false
    name_fr: 
      label: French
      $type: textField
      i18n: false
  layout: 
    tabs: 
      - name: tabTranslation
        label: URL Translation
        fields: 
          - name: name_de
          - name: name_fr

Then I get this: