config: Fix env override of slices

Fixes #13707

config/allconfig/allconfig_integration_test.go

@@ -357,3 +357,25 @@ All.
 
 	b.Assert(b.H.Conf.DefaultContentLanguage(), qt.Equals, "sv")
 }
+
+func TestDefaultConfigEnvDisableLanguagesIssue13707(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableLanguages = []
+[languages]
+[languages.en]
+weight = 1
+[languages.nn]
+weight = 2
+[languages.sv]
+weight = 3
+`
+
+	b := hugolib.Test(t, files, hugolib.TestOptWithConfig(func(conf *hugolib.IntegrationTestConfig) {
+		conf.Environ = []string{`HUGO_DISABLELANGUAGES=sv nn`}
+	}))
+
+	b.Assert(len(b.H.Sites), qt.Equals, 1)
+}

config/allconfig/load.go

@@ -233,17 +233,18 @@ func (l configLoader) applyOsEnvOverrides(environ []string) error {
 
 		if existing != nil {
 			val, err := metadecoders.Default.UnmarshalStringTo(env.Value, existing)
-			if err != nil {
-				continue
-			}
-
+			if err == nil {
+				val = l.envValToVal(env.Key, val)
 				if owner != nil {
 					owner[nestedKey] = val
 				} else {
 					l.cfg.Set(env.Key, val)
 				}
-		} else {
-			if nestedKey != "" {
+				continue
+			}
+		}
+
+		if owner != nil && nestedKey != "" {
 			owner[nestedKey] = env.Value
 		} else {
 			var val any
@@ -255,18 +256,28 @@ func (l configLoader) applyOsEnvOverrides(environ []string) error {
 					val = v
 				}
 			}
+
 			if val == nil {
 				// A string.
 				val = l.envStringToVal(key, env.Value)
 			}
 			l.cfg.Set(key, val)
 		}
-		}
+
 	}
 
 	return nil
 }
 
+func (l *configLoader) envValToVal(k string, v any) any {
+	switch v := v.(type) {
+	case string:
+		return l.envStringToVal(k, v)
+	default:
+		return v
+	}
+}
+
 func (l *configLoader) envStringToVal(k, v string) any {
 	switch k {
 	case "disablekinds", "disablelanguages":