Call the default language preference setting "Use system setting".

That more correctly describes it - it doesn't do any form of automatic
detection of anything, it just picks up the system settings from
wherever Qt picks them up.

Change-Id: I62d40719728cc9735e3b8f3e4202b7e61fc3f43b
Reviewed-on: https://code.wireshark.org/review/5165
Reviewed-by: Guy Harris <guy@alum.mit.edu>