[Impact]
When working with ubuntu core or ubuntu core desktop, neither */etc/default/locale* nor */etc/default/keyboard* are modifiable, so it's not possible to set the global keyboard or the global language. This is required to allow to set the GDM language, and the default one during installation.
The first half of the solution is to create the folder */etc/writable/default*, and make soft-links from */etc/default/locale* to */etc/writable/default/locale* and from */etc/default/keyboard* to */etc/writable/default/keyboard*, just like it is already being done with */etc/hostname*, */etc/issue*, */etc/localtime*, */etc/motd* and , */etc/timezone*.
This solution, unfortunately, isn't complete. Although any application that just reads the files will work, not all of the applications that write to them will; specifically the systemd utilities that set the contents for those files, because they don't open the file directly; instead, they create first the new file in the same folder than the old one, fill its contents, and only then delete the old one and rename the new one. To solve this, systemd in Ubuntu already has several patches that detect if a file is a soft-link, in which case it replaces the old path with the destination one.
Currently I have in place a patch for Ubuntu Core Desktop that implements both changes for both */etc/default/locale* and */etc/default/keyboard*.
[Test plan]
Using *sudo localectl set-locale xx_YY.UTF-8* in an Ubuntu Core or Ubuntu Core Desktop admin terminal must change the locale to the specified one, which can be checked by reading the */etc/default/locale* file. Also, *localectl* must return the new locale.
Using *sudo dbus-send --system --print-reply --dest=org.freedesktop.locale1 /org/freedesktop/locale1 org.freedesktop.locale1.SetX11Keyboard string:XX string:pc10Y string: string: boolean:true boolean:false" must change the */etc/default/keyboard* file to layout XX and model PC10Y (being Y either 1, 2, 4 or 5). Reading the file allows to check it. Also, *localectl status* must return the layout and model values in "X11 Layout" and "X11 Model" entries.
[Where problems could occur]
In general, applications just read the content of the file and use the DBus interface to set the locale, so only those applications that modify by themselves the */etc/default/keyboard* and/or */etc/default/locale* would present a problem, in which case they would require specific patches. Anyway, those applications neither would work with the current state (with those files in a read-only filesystem).
[Other info]
For Noble, this will be addressed when we merge systemd v255 from Debian. This is only needed on core, so we don't need to fix for Mantic or Lunar.
I have a patch that fixes this. We are already using it in ubuntu core desktop. I'm preparing to upload it to the GIT repo.