diff --git a/backend/src/Squidex.Domain.Apps.Entities/Contents/ContentExtensions.cs b/backend/src/Squidex.Domain.Apps.Entities/Contents/ContentExtensions.cs
index eb651eb67..9afdf66de 100644
--- a/backend/src/Squidex.Domain.Apps.Entities/Contents/ContentExtensions.cs
+++ b/backend/src/Squidex.Domain.Apps.Entities/Contents/ContentExtensions.cs
@@ -135,10 +135,7 @@ namespace Squidex.Domain.Apps.Entities.Contents
 
                 foreach (var iso2Code in value.Split(Separators, StringSplitOptions.RemoveEmptyEntries))
                 {
-                    if (Language.TryGetLanguage(iso2Code.Trim(), out var language))
-                    {
-                        languages.Add(language);
-                    }
+                    languages.Add(iso2Code);
                 }
 
                 return languages;
diff --git a/backend/tools/TestSuite/TestSuite.ApiTests/ContentLanguageTests.cs b/backend/tools/TestSuite/TestSuite.ApiTests/ContentLanguageTests.cs
index 44c15bd33..6c7853564 100644
--- a/backend/tools/TestSuite/TestSuite.ApiTests/ContentLanguageTests.cs
+++ b/backend/tools/TestSuite/TestSuite.ApiTests/ContentLanguageTests.cs
@@ -43,6 +43,7 @@ namespace TestSuite.ApiTests
         [Theory]
         [InlineData("de", "Hallo")]
         [InlineData("en", "Hello")]
+        [InlineData("custom", "Custom")]
         public async Task Should_flatten_language(string code, string expected)
         {
             // STEP 1: Create content
@@ -51,7 +52,8 @@ namespace TestSuite.ApiTests
                 Localized = new Dictionary<string, string>
                 {
                     ["de"] = "Hallo",
-                    ["en"] = "Hello"
+                    ["en"] = "Hello",
+                    ["custom"] = "Custom"
                 }
             }, ContentCreateOptions.AsPublish);
 
diff --git a/backend/tools/TestSuite/TestSuite.Shared/Fixtures/CreatedAppFixture.cs b/backend/tools/TestSuite/TestSuite.Shared/Fixtures/CreatedAppFixture.cs
index afe323c47..09416f37a 100644
--- a/backend/tools/TestSuite/TestSuite.Shared/Fixtures/CreatedAppFixture.cs
+++ b/backend/tools/TestSuite/TestSuite.Shared/Fixtures/CreatedAppFixture.cs
@@ -47,6 +47,21 @@ namespace TestSuite.Fixtures
                     }
                 }
 
+                try
+                {
+                    await Apps.PostLanguageAsync(AppName, new AddLanguageDto
+                    {
+                        Language = "custom"
+                    });
+                }
+                catch (SquidexManagementException ex)
+                {
+                    if (ex.StatusCode != 400)
+                    {
+                        throw;
+                    }
+                }
+
                 return true;
             });
         }