Save GUI language as string instead of wxLanguage enum

When 5.0-211 updated wxWidgets to 3.1.0, some entries in the
wxLanguage enum were moved and added, changing the wxLanguage
values. Because we save Dolphin's interface language to disk
as a wxLanguage, the language you have set will mean something
different depending on whether you have the updated wx version
or not. For instance, setting the language to English with the
updated version and then using an older version will make
Dolphin use Dutch. Because we can't rely on the enum anymore,
I'm replacing the "Language" setting with a "LanguageCode"
setting that uses standard ISO 639 codes.

Source/Core/Core/ConfigManager.h

@@ -210,7 +210,7 @@ struct SConfig : NonCopyable
   std::string m_bba_mac;
 
   // interface language
-  int m_InterfaceLanguage;
+  std::string m_InterfaceLanguage;
   float m_EmulationSpeed;
   bool m_OCEnable;
   float m_OCFactor;