[NUI.Gadget] Fix NUIGadgetResourceManager (#6070)

If getting the resource manager is failed, the NUIGadgeResourceManager
tries to get the resource manager from the TwoLetterISOLanguageName of
the CultureInfo.

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>

src/Tizen.NUI.Gadget/Tizen.NUI/NUIGadgetResourceManager.cs

@@ -104,6 +104,11 @@ public string GetString(string name, CultureInfo cultureInfo)
             try
             {
                 var resourceManager = GetResourceManager(cultureInfo.Name);
+                if (resourceManager == null)
+                {
+                    resourceManager = GetResourceManager(cultureInfo.TwoLetterISOLanguageName);
+                }
+
                 if (resourceManager != null)
                 {
                     result = resourceManager.GetString(name, cultureInfo);