Don't know where to start here but will start with new registered users.
» Control Panel » Registration Settings
"Default Group(s) for New Members: " Should have a selection for what the default group new registrations are assigned to.
It should have been assigned already from ver7.4.

When you say everyone that registered are you saying registered after the upgrade or before the up grade.
If it is before the upgrade you have a problem and may need a trouble ticket. If after the upgrade by setting the default usere group that problem should go away.

User language is set during the install but can be set at:
» Control Panel » Languages
You can only use one language. Check if it is selected.

Default User post layout:
» Control Panel » Display Options
Topic Display Options
Should set the deault settings.
» Control Panel » Registration Settings
Flat or Threaded:
Allows the user to change the settings.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers