Add simple locking of resourceManagers dictionary for thread safety

osu.Game/Localisation/ResourceManagerLocalisationStore.cs

@@ -31,6 +31,8 @@ namespace osu.Game.Localisation
             string ns = split[0];
             string key = split[1];
 
+            lock (resourceManagers)
+            {
                 if (!resourceManagers.TryGetValue(ns, out var manager))
                     resourceManagers[ns] = manager = new ResourceManager(ns, GetType().Assembly);
 
@@ -45,6 +47,7 @@ namespace osu.Game.Localisation
                     return null;
                 }
             }
+        }
 
         public Task<string> GetAsync(string lookup)
         {