Policy Plus - ADML language override

Normally Policy Plus detects the user's locale and tries to load ADML files for that language. A Policy Plus user requested the ability to change that behavior, which would be helpful when looking for a policy mentioned in a guide written in another language. So I added a simple dialog to change the preferred language:

If a preferred language code has been set, Policy Plus will use that instead of the locale name.

This change is live on GitHub.