Validate language when starting up SharpDevelop.

src/Main/Base/Project/Src/Gui/WorkbenchSingleton.cs

@@ -62,7 +62,9 @@ namespace ICSharpCode.SharpDevelop.Gui
 		public static void InitializeWorkbench(IWorkbench workbench, IWorkbenchLayout layout)
 		{
 			WorkbenchSingleton.workbench = workbench;
-
+			
+			LanguageService.ValidateLanguage();
+			
 			DisplayBindingService.InitializeService();
 			LayoutConfiguration.LoadLayoutConfiguration();
 			FileService.InitializeService();

src/Main/Base/Project/Src/Services/Language/LanguageService.cs

@@ -58,5 +58,13 @@ namespace ICSharpCode.SharpDevelop
 			}
 			LanguageService.languages = languages.AsReadOnly();
 		}
+		
+		/// <summary>
+		/// Ensures that the active language exists
+		/// </summary>
+		public static void ValidateLanguage()
+		{
+			ResourceService.Language = GetLanguage(ResourceService.Language).Code;
+		}
 	}
 }

src/Main/Core/Project/Src/Services/ResourceService/ResourceService.cs

@@ -42,7 +42,9 @@ namespace ICSharpCode.Core
 				return PropertyService.Get(uiLanguageProperty, Thread.CurrentThread.CurrentUICulture.Name);
 			}
 			set {
-				PropertyService.Set(uiLanguageProperty, value);
+				if (Language != value) {
+					PropertyService.Set(uiLanguageProperty, value);
+				}
 			}
 		}