Fail on invalid defaultContentLanguage

Fixes gohugoio#11044

config/allconfig/alldecoders.go

@@ -60,7 +60,14 @@ var allDecoderSetups = map[string]decodeWeight{
 		key:    "",
 		weight: -100, // Always first.
 		decode: func(d decodeWeight, p decodeConfig) error {
-			return mapstructure.WeakDecode(p.p.Get(""), &p.c.RootConfig)
+			if err := mapstructure.WeakDecode(p.p.Get(""), &p.c.RootConfig); err != nil {
+				return err
+			}
+
+			// This need to match with Lang which is always lower case.
+			p.c.RootConfig.DefaultContentLanguage = strings.ToLower(p.c.RootConfig.DefaultContentLanguage)
+
+			return nil
 		},
 	},
 	"imaging": {
@@ -263,6 +270,18 @@ var allDecoderSetups = map[string]decodeWeight{
 				return err
 			}
 
+			// Validate defaultContentLanguage.
+			var found bool
+			for lang := range p.c.Languages {
+				if lang == p.c.DefaultContentLanguage {
+					found = true
+					break
+				}
+			}
+			if !found {
+				return fmt.Errorf("config value %q for defaultContentLanguage does not match any language definition", p.c.DefaultContentLanguage)
+			}
+
 			return nil
 		},
 	},

hugolib/config_test.go

@@ -1122,4 +1122,32 @@ Foo: {{ site.Params.foo }}|
 		b.Assert(err.Error(), qt.Contains, "no languages")
 	})
 
+	// Issue 11044
+	t.Run("invalid defaultContentLanguage", func(t *testing.T) {
+
+		files := `
+-- hugo.toml --
+baseURL = "https://example.org"
+defaultContentLanguage = "sv"
+
+[languages]
+[languages.en]
+languageCode = "en"
+languageName = "English"
+weight = 1
+
+	
+		
+	`
+		b, err := NewIntegrationTestBuilder(
+			IntegrationTestConfig{
+				T:           t,
+				TxtarString: files,
+			},
+		).BuildE()
+
+		b.Assert(err, qt.IsNotNil)
+		b.Assert(err.Error(), qt.Contains, "defaultContentLanguage does not match any language definition")
+	})
+
 }