Revise how the language system works.
On Win32, the Language menubar is eliminated in favour of displaying a NewLanguageScreen on screens that are of type UIDialogScreenWithBackground/UIScreenWithBackground; UIScreen inheriting screens (e.g. GamePauseScreen) still require manual additions.
Merge pull request #4419 from thedax/reviseLanguageSwitching

Win32: Revise how the language system/menu works.