Binary package hint: gnome-settings-daemon
As mentioned in bug #328980 comment #5, it would be a good idea to have users include their ~/.xsession-errors when reporting errors with their keybord konfiguration. This is because errors from xkbcomp end up in that file. These would be particularly useful in diagnosing and resolving bugs like bug #67188.
I therefore would ask you to add that file or its tail to the list of information requested by the activation_error function in plugins/keyboard/gsd-keyboard-xkb.c. Requesting the result of "tail -n50 ~/.xsession-errors" has the benefit of avoiding excessively large posts. It also reuses the wording of the other requests, making it simple to adjust most translations even if you don't speak the language. Requesting only command output, as opposed to command output and file contents, should be easier for users as well.
Unfortunately I don't think there is a reliable word to grep for, so we'll have to request more information unless we want users to look for appropriate error messages themselves.
OK, this patch does implement the request as outlined above.
I even adjusted the message files, except for the Azerbaijani (az) one, as that uses different wording for the two existing lines, and I have no clue why that's the case. Some other languages were changed from fixed command strings to %s as in the message template.
None of this got tested by me, but I expect things to work nevertheless. You might, however, want to give one of the RTL languages, e.g. Arabic (ar), a try, just to make sure my editor didn't garble something.