Use the glibc locale database to add GDateTime strings only for those locales
that had not specifically added them for themselves.
Some locales have different translations from what is in the C library and we
leave those alone with this commit.
The Asturian, French, Norwegian Nynorsk and Spanish translations
incorrectly translated "MB" and friends to "MiB" (french: "Mio"), etc.
This was in response to the incorrect use of "MB" in the (now
deprecated) g_format_size_for_display() function.
These strings are now used (correctly) in g_format_size(), so I have
updated the translations accordingly.
Additionally, the Norwegian Nynorsk translation was incorrectly
translating several larger units to "KiB", so that has been corrected as
well.