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/OsuGame.cs

@@ -628,6 +628,14 @@ namespace osu.Game
 
             foreach (var language in Enum.GetValues(typeof(Language)).OfType<Language>())
             {
+#if DEBUG
+                if (language == Language.DebugLocalisation)
+                {
+                    Localisation.AddLanguage(Language.DebugLocalisation.ToString(), new DebugLocalisationStore());
+                    continue;
+                }
+#endif
+
                 string cultureCode = language.ToCultureCode();
 
                 try