Add language service

Oqtane.Client/Services/Interfaces/ILanguageService.cs

@@ -0,0 +1,17 @@
+using Oqtane.Models;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Oqtane.Services
+{
+    public interface ILanguageService
+    {
+        Task<List<Language>> GetLanguagesAsync(int siteId);
+
+        Task<Language> GetLanguageAsync(int languageId);
+
+        Task<Language> AddLanguageAsync(Language language);
+
+        Task DeleteLanguageAsync(int languageId);
+    }
+}