fix(ubuntu_localizations): use fallback if locale is 'POSIX' or 'und' (#476)

If `Intl.defaultLocale` is initialized with `und` (undefined) or `POSIX`
localizing a `DateTime` with `DateFormat` (and possibly other things)
will throw an exception. See
ubuntu/app-center#1659 for more details.

A simple workaround has been suggested
[here](ubuntu/app-center#1884 (comment)).

---------

Co-authored-by: ashuntu <ashton.nelson@canonical.com>

Author: d-loose

packages/ubuntu_localizations/lib/src/localizations.dart

@@ -30,7 +30,10 @@ class GlobalUbuntuLocalizations {
 /// * [Intl.defaultLocale]
 /// * [Intl.systemLocale]
 Future<void> initDefaultLocale([String? locale]) async {
-  Intl.defaultLocale = locale ?? await findSystemLocale();
+  Intl.defaultLocale = locale ??
+      await findSystemLocale()
+          // Fallback to 'en' if the system locale is 'POSIX' or undefined.
+          .then((l) => l == 'POSIX' || l == 'und' ? 'en' : l);
 }
 
 /// A localized language and its locale.