Allowed delete of langage via services even if used as a fall-back for other languages, by setting references to null before deleting

src/Umbraco.Core/Persistence/Dtos/LanguageDto.cs

@@ -45,6 +45,7 @@ namespace Umbraco.Core.Persistence.Dtos
         /// </summary>
         [Column("fallbackLanguageId")]
         [ForeignKey(typeof(LanguageDto), Column = "id")]
+        [Index(IndexTypes.NonClustered)]
         [NullSetting(NullSetting = NullSettings.Null)]
         public int? FallbackLanguageId { get; set; }
     }

src/Umbraco.Core/Persistence/Repositories/Implement/LanguageRepository.cs

@@ -177,7 +177,7 @@ namespace Umbraco.Core.Persistence.Repositories.Implement
 
         protected override void PersistDeletedItem(ILanguage entity)
         {
-            //we need to validate that we can delete this language
+            // We need to validate that we can delete this language
             if (entity.IsDefaultVariantLanguage)
                 throw new InvalidOperationException($"Cannot delete the default language ({entity.IsoCode})");
 
@@ -185,9 +185,12 @@ namespace Umbraco.Core.Persistence.Repositories.Implement
             if (count == 1)
                 throw new InvalidOperationException($"Cannot delete the default language ({entity.IsoCode})");
 
+            // We need to remove any references to the language if it's being used as a fall-back from other ones
+            Database.Execute(Sql().Update<LanguageDto>(u => u.Set(x => x.FallbackLanguageId, null)).Where<LanguageDto>(x => x.FallbackLanguageId == entity.Id));
+
             base.PersistDeletedItem(entity);
 
-            //Clear the cache entries that exist by key/iso
+            // Clear the cache entries that exist by key/iso
             IsolatedCache.ClearCacheItem(RepositoryCacheKeys.GetKey<ILanguage>(entity.IsoCode));
             IsolatedCache.ClearCacheItem(RepositoryCacheKeys.GetKey<ILanguage>(entity.CultureName));
         }

src/Umbraco.Core/Services/Implement/LocalizationService.cs

@@ -393,8 +393,7 @@ namespace Umbraco.Core.Services.Implement
                     return;
                 }
 
-                //NOTE: There isn't any constraints in the db, so possible references aren't deleted
-
+                // NOTE: Other than the fall-back language, there aren't any other constraints in the db, so possible references aren't deleted
                 _languageRepository.Delete(language);
                 deleteEventArgs.CanCancel = false;
 

src/Umbraco.Tests/Persistence/Repositories/LanguageRepositoryTest.cs

@@ -318,6 +318,30 @@ namespace Umbraco.Tests.Persistence.Repositories
             }
         }
 
+        [Test]
+        public void Can_Perform_Delete_On_LanguageRepository_With_Language_Used_As_Fallback()
+        {
+            // Arrange
+            var provider = TestObjects.GetScopeProvider(Logger);
+            using (var scope = provider.CreateScope())
+            {
+                // Add language to delete as a fall-back language to another one
+                var repository = CreateRepository(provider);
+                var languageToFallbackFrom = repository.Get(5);
+                languageToFallbackFrom.FallbackLanguageId = 1;
+                repository.Save(languageToFallbackFrom);
+
+                // Act
+                var languageToDelete = repository.Get(1);
+                repository.Delete(languageToDelete);
+
+                var exists = repository.Exists(1);
+
+                // Assert
+                Assert.That(exists, Is.False);
+            }
+        }
+
         [Test]
         public void Can_Perform_Exists_On_LanguageRepository()
         {

src/Umbraco.Tests/Services/LocalizationServiceTests.cs

@@ -192,6 +192,20 @@ namespace Umbraco.Tests.Services
             Assert.Null(language);
         }
 
+        [Test]
+        public void Can_Delete_Language_Used_As_Fallback()
+        {
+            var danish = ServiceContext.LocalizationService.GetLanguageByIsoCode("da-DK");
+            var norwegian = new Language("nb-NO") { CultureName = "Norwegian", FallbackLanguageId = danish.Id };
+            ServiceContext.LocalizationService.Save(norwegian, 0);
+            var languageId = danish.Id;
+
+            ServiceContext.LocalizationService.Delete(danish);
+
+            var language = ServiceContext.LocalizationService.GetLanguageById(languageId);
+            Assert.Null(language);
+        }
+
         [Test]
         public void Can_Create_DictionaryItem_At_Root()
         {