Fix the logic when to display the "language changed" warning

If the user had never set up the language selection they could end up
getting the "language changed, restart required" warning even if they
didn't touch the language setting at all.

This fixes that issue by assuming that UseSystemLanguage is true if the
setting is undefined and only comparing the selected language if that
selection actually matters (i.e., UseSystemLanguage is false).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>