Default to en-US for missing core translations

Emby.Server.Implementations/Localization/LocalizationManager.cs

@@ -459,6 +459,9 @@ namespace Emby.Server.Implementations.Localization
         private async Task CopyInto(IDictionary<string, string> dictionary, string resourcePath)
         {
             using (var stream = _assembly.GetManifestResourceStream(resourcePath))
+            {
+                // If a Culture doesn't have a translation the stream will be null and it defaults to en-us further up the chain
+                if (stream != null)
                 {
                     var dict = await _jsonSerializer.DeserializeFromStreamAsync<Dictionary<string, string>>(stream);
 
@@ -468,6 +471,7 @@ namespace Emby.Server.Implementations.Localization
                     }
                 }
             }
+        }
 
         private static string GetResourceFilename(string culture)
         {