Drupal 8 migrate language: switch all nodes to the default lang
You need to change your language selection criteria in this admin setting:

Admin -> Configuration -> Regional &amp; Language -> Language / Detection and Selection Tab

You can turn off or configure the URL settings as you want.  With no standard detection, the default language should be shown.

You probably want to turn on the administration language detection option.  This allows admin users to select their preferred admin language.