mode: oem is specific to the whitelabel.yaml that the Flutter installer uses to have the user creation page skipped. For an autoinstall.yaml, the absence of an identity clause skips the use creation. So you can remove the mode clause from your autoinstall.yaml file.
There are no additional languages available because the installer removes all but the selected language post install from the target system. If you are installing via an autoinstall.yaml, it will be decided by this clause. I’ve appended to the docs to show how you can have additional language packs installed to appear in Gnome Initial Setup.
The empty and non-functional screen is not something I’ve been able to replicate, could you tell me more about your setup and share any logs you got? Thanks.