Fix it so languageCode on top level config still works

This is common for monolingual sites, and we broke this in Hugo 0.112.4.

Fixes gohugoio#11037

config/allconfig/alldecoders.go

@@ -241,8 +241,24 @@ var allDecoderSetups = map[string]decodeWeight{
 		key: "languages",
 		decode: func(d decodeWeight, p decodeConfig) error {
 			var err error
-			p.c.Languages, err = langs.DecodeConfig(p.p.GetStringMap(d.key))
-			return err
+			m := p.p.GetStringMap(d.key)
+			if len(m) == 1 {
+				// In v0.112.4 we moved this to the language config, but it's very commmon for mono language sites to have this at the top level.
+				var first maps.Params
+				for _, v := range m {
+					first = v.(maps.Params)
+					break
+				}
+				if _, found := first["languagecode"]; !found {
+					first["languagecode"] = p.p.GetString("languagecode")
+				}
+			}
+			p.c.Languages, err = langs.DecodeConfig(m)
+			if err != nil {
+				return err
+			}
+
+			return nil
 		},
 	},
 	"cascade": {

hugolib/config_test.go

@@ -1044,3 +1044,25 @@ HTACCESS.
 	b.AssertFileContent("public/.htaccess", "HTACCESS")
 
 }
+
+func TestConfigLanguageCodeTopLevel(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+languageCode = "en-US"
+-- layouts/index.html --
+LanguageCode: {{ .Site.LanguageCode }}|{{ site.Language.LanguageCode }}|
+
+	
+`
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).Build()
+
+	b.AssertFileContent("public/index.html", "LanguageCode: en-US|en-US|")
+
+}