langs/i18n: Improve default content language fallback

The fallback order for translations is now:

1. Current language's locale (e.g., pt-BR → pt-br.toml)
2. Current language's key (e.g., pt → pt.toml)
3. Default language's locale (e.g., es-AR → es-ar.toml) ← new
4. Default language's key (e.g., es → es.toml)

Closes #14243

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

hugolib/menu_test.go

@@ -567,7 +567,7 @@ menu: main
 
 	b.AssertFileContent("public/en/index.html", `<a href="/en/p1/">p1</a><a href="/en/p2/">p2</a>`)
 	b.AssertFileContent("public/fr/index.html", `<a href="/fr/p1/">p1</a>`)
-	b.AssertLogContains("! WARN")
+	b.AssertLogContains("! duplicate menu entry")
 }
 
 func TestSectionPagesIssue12399(t *testing.T) {

langs/i18n/translationProvider.go

@@ -21,6 +21,7 @@ import (
 	"strings"
 
 	"github.com/gohugoio/hugo/common/paths"
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/parser/metadecoders"
 
@@ -150,17 +151,29 @@ func (tp *TranslationProvider) CloneResource(dst, src *deps.Deps) error {
 	return nil
 }
 
-// getTranslateFunc returns the translation function for the language in Deps.
-// We first try the locale (e.g. "en-US"), then the language key (e.g. "en").
-func (tp *TranslationProvider) getTranslateFunc(dst *deps.Deps) func(ctx context.Context, translationID string, templateData any) string {
-	l := dst.Conf.Language().(*langs.Language)
-	if locale := l.Locale(); locale != "" {
-		if fn, ok := tp.t.Lookup(strings.ToLower(locale)); ok {
-			return fn
+func defaultLanguage(conf config.AllProvider) *langs.Language {
+	key := conf.DefaultContentLanguage()
+	for _, l := range conf.Languages().(langs.Languages) {
+		if l.Lang == key {
+			return l
 		}
 	}
-	// Func will fall back to the default language if not found.
-	return tp.t.Func(l.Lang)
+	return conf.Language().(*langs.Language)
+}
+
+// getTranslateFunc returns the translation function for the language in Deps.
+// The lookup order is: current locale, current key, default locale, default key.
+func (tp *TranslationProvider) getTranslateFunc(dst *deps.Deps) func(ctx context.Context, translationID string, templateData any) string {
+	current := dst.Conf.Language().(*langs.Language)
+	defaultLang := defaultLanguage(dst.Conf)
+	for _, l := range []*langs.Language{current, defaultLang} {
+		for _, key := range []string{strings.ToLower(l.Locale()), l.Lang} {
+			if fn, ok := tp.t.Lookup(key); ok {
+				return fn
+			}
+		}
+	}
+	return tp.t.Func("en")
 }
 
 func errWithFileContext(inerr error, r *source.File) error {

langs/languages_integration_test.go

@@ -326,3 +326,55 @@ language: {{ site.Language.Name }} file: {{ T "file" }}|
 	b.AssertFileContent("public/de/index.html", "language: de file: de|")
 	b.AssertFileContent("public/de-de/index.html", "language: de-de file: de-de|")
 }
+
+func TestDefaultContentLanguageFallback14243(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+defaultContentLanguage = 'es'
+defaultContentLanguageInSubdir = true
+
+[languages.es]
+locale = 'es-AR'
+weight = 1
+
+[languages.pt]
+locale = 'pt-BR'
+weight = 2
+-- layouts/home.html --
+{{ T "foo"}}|
+-- i18n/es-ar.toml --
+foo = 'foo es-ar'
+-- i18n/es.toml --
+foo = 'foo es'
+-- i18n/pt-br.toml --
+foo = 'foo pt-br'
+-- i18n/pt.toml --
+foo = 'foo pt'
+`
+
+	b := hugolib.Test(t, files)
+	b.AssertFileContent("public/pt/index.html", "foo pt-br|")
+
+	files = strings.ReplaceAll(files, "i18n/pt-br.toml", "unused-a.toml")
+
+	b = hugolib.Test(t, files)
+	b.AssertFileContent("public/pt/index.html", "foo pt|")
+
+	files = strings.ReplaceAll(files, "i18n/pt.toml", "unused-b.toml")
+
+	b = hugolib.Test(t, files)
+	b.AssertFileContent("public/pt/index.html", "foo es-ar|")
+
+	files = strings.ReplaceAll(files, "i18n/es-ar.toml", "unused-c.toml")
+
+	b = hugolib.Test(t, files)
+	b.AssertFileContent("public/pt/index.html", "foo es|")
+
+	files = strings.ReplaceAll(files, "i18n/es.toml", "unused-d.toml")
+
+	b = hugolib.Test(t, files)
+	b.AssertFileContent("public/pt/index.html", "|")
+}