Win32 UI: Rewrite most of the menu translation code to be cleaner and simpler.

This does the following:
* Vastly simplifies the TranslateMenu* code.
* Fixes a bug where in-game only menu items become enabled after selecting a language when we're in various NewUI menus.
* Renames Config::ilanguage to Config::iLanguage for consistency with other Config variables.
* Updates Config::iLanguage in CreateLanguageMenu instead of in the WndProc.