Mod Locale Loading

[ColinTimBarndt]

This PR adds support for mods to provide their own locale JSON files as embedded resource the same way how RML is already doing it. For doing this, I have generalized the LocaleLoader of RML to also consider mod assemblies for discovery.

Some other notable changes:

• Use CultureInfo for parsing language code.
  • Example: "ab-CD" language code will result in searches for "Locale/ab-cd.json" and "Locale/ab.json" if the first one does not exist.
• Fixed error message: Embedded locale resource for 'en-US' not found. Using fallback.
  • When the game loads locales for the first time (before settings are applied) the locale is set to "en-US" and not "en".