Add debug language to help with localisation efforts

The idea is to allow a developer to immediately see which text on a
component or screen has already got localisation support. It can be a
bit of a challenge to see this when creating a new component that
doesn't yet have any translations populated.

Curious to hear thoughts on this. I could see it working very well as a
visual tests checkbox (implemented at o!f side), potentially in addition
to having this at the game level, or replacing this PR.

osu.Game/Localisation/Language.cs

@@ -110,6 +110,11 @@ namespace osu.Game.Localisation
         // zh_hk,
 
         [Description(@"繁體中文（台灣）")]
-        zh_hant
+        zh_hant,
+
+#if DEBUG
+        [Description(@"Debug (show raw keys)")]
+        DebugLocalisation
+#endif
     }
 }