Answer files (or Unattend files) can be used to modify Windows settings in your images during Setup. You can also create settings that trigger scripts in your images that run after the first user creates their account and picks their default language.
To find the proper language ID, use Ctrl+Shift+P (or View -> Command Palette from the menu) and then type Change Language Mode. You can see the language ID in the list, e.g. type docker to find the language ID for Docker files (dockerfile). In the first entry in the example above, .thor is the file ending, ruby is the language ID.