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/DebugLocalisationStore.cs

@@ -0,0 +1,30 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using osu.Framework.Localisation;
+
+namespace osu.Game.Localisation
+{
+    public class DebugLocalisationStore : ILocalisationStore
+    {
+        public string Get(string lookup) => $@"[[{lookup.Substring(lookup.LastIndexOf('.') + 1)}]]";
+
+        public Task<string> GetAsync(string lookup, CancellationToken cancellationToken = default) => Task.FromResult(Get(lookup));
+
+        public Stream GetStream(string name) => throw new NotImplementedException();
+
+        public IEnumerable<string> GetAvailableResources() => throw new NotImplementedException();
+
+        public CultureInfo EffectiveCulture { get; } = new CultureInfo(@"debug");
+
+        public void Dispose()
+        {
+        }
+    }
+}

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
     }
 }

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