langs: Use Language.Locale as primary localization key

Localization now uses Language.Locale as the golocales lookup key,
falling back to Language.Lang, then defaultContentLanguage, then "en".

Closes #9109

langs/language.go

@@ -79,11 +79,13 @@ func (l *Language) IsDefault() bool {
 
 // NewLanguage creates a new language.
 func NewLanguage(lang, defaultContentLanguage, timeZone string, languageConfig LanguageConfig, logger loggers.Logger) (*Language, error) {
-	translator := golocales.New(lang)
+	var translator golocales.Translator
-	if translator == nil {
+	for _, key := range []string{languageConfig.Locale, lang, defaultContentLanguage, "en"} {
-		translator = golocales.New(defaultContentLanguage)
+		if key == "" {
-		if translator == nil {
+			continue
-			translator = golocales.New("en")
+		}
+		if translator = golocales.New(key); translator != nil {
+			break
 		}
 	}
 

langs/languages_integration_test.go

@@ -215,3 +215,84 @@ TITLE: {{ .Title }} RELPERMALINK: {{ .RelPermalink }}|
 		"b (en)|TITLE: P1 (en) RELPERMALINK: /en/p1/|",
 	)
 }
+
+func TestLocaleAsLocalizationKeyIssue9109(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+defaultContentLanguage = 'de'
+defaultContentLanguageInSubdir = true
+
+[languages.de]
+weight = 1
+
+[languages.en]
+locale = 'en-US'
+weight = 2
+
+[languages.fr]
+locale = 'bogus'
+weight = 3
+
+[languages.nn]
+weight = 4
+
+[languages.xx]
+locale = 'bogus'
+weight = 5
+
+[languages.zh-cn]
+locale = 'zh-Hans'
+weight = 6
+-- layouts/home.html --
+Time: {{ "2026-04-01T20:11:31+08:00" | time.Format ":date_long" }}|
+FormatAccounting: {{ 512.5032 | lang.FormatAccounting 2 "USD" }}|
+FormatCurrency: {{ 512.5032 | lang.FormatCurrency 2 "USD" }}|
+`
+
+	b := hugolib.Test(t, files)
+
+	// de: no locale => localize with lang.
+	b.AssertFileContent("public/de/index.html",
+		`Time: 1. April 2026|`,
+		"FormatAccounting: 512,50\u00a0$|",
+		"FormatCurrency: 512,50\u00a0$|",
+	)
+
+	// en: valid locale => localize with locale.
+	b.AssertFileContent("public/en/index.html",
+		`Time: April 1, 2026|`,
+		`FormatAccounting: $512.50|`,
+		`FormatCurrency: $512.50|`,
+	)
+
+	// fr: invalid locale => localize with lang.
+	b.AssertFileContent("public/fr/index.html",
+		`Time: 1 avril 2026|`,
+		"FormatAccounting: 512,50\u00a0$US|",
+		"FormatCurrency: 512,50\u00a0$US|",
+	)
+
+	// nn: no locale => localize with lang.
+	b.AssertFileContent("public/nn/index.html",
+		`Time: 1. april 2026|`,
+		"FormatAccounting: 512,50\u00a0USD|",
+		"FormatCurrency: 512,50\u00a0USD|",
+	)
+
+	// xx: invalid locale + invalid lang => localize with defaultContentLanguage.
+	b.AssertFileContent("public/xx/index.html",
+		`Time: 1. April 2026|`,
+		"FormatAccounting: 512,50\u00a0$|",
+		"FormatCurrency: 512,50\u00a0$|",
+	)
+
+	// zh-cn: invalid lang, but valid locale => localize with locale.
+	b.AssertFileContent("public/zh-cn/index.html",
+		`Time: 2026年4月1日|`,
+		`FormatAccounting: US$512.50|`,
+		`FormatCurrency: US$512.50|`,
+	)
+}