Ubuntu’s “Keyboard Preferences” window currently lets you set your own keyboard layout. We want to be able to set a default layout that applies to the guest account, new user accounts when created, and the login screen.
We can do this by rearranging the bottom of the “Layouts” tab so that there is an “Apply System-Wide…” button next to the "Reset" button.
Erratum: “Use System-Wide…” should be “Apply System-Wide…”.
The “Apply System-Wide…” button should apply your current personal settings as the system-wide ones. In doing so, it should produce a PolicyKit alert for authenticating as someone with permission to change the default settings.