From d5060197f529f68774c173769d98283c04755f95 Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Thu, 2 Apr 2026 04:41:24 +0000
Subject: [PATCH] feat(i18n): add base language message fallback

Co-Authored-By: Virgil <virgil@lethean.io>
---
 service.go      |  8 +++++++-
 service_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/service.go b/service.go
index 0b87025..969d72d 100644
--- a/service.go
+++ b/service.go
@@ -375,11 +375,17 @@ func (s *Service) T(messageID string, args ...any) string {
 	return result
 }
 
-// resolveDirect performs exact-key lookup in the current language and fallback language.
+// resolveDirect performs exact-key lookup in the current language, its base
+// language tag, and then the configured fallback language.
 func (s *Service) resolveDirect(messageID string, data any) string {
 	if text := s.tryResolve(s.currentLang, messageID, data); text != "" {
 		return text
 	}
+	if base := baseLanguageTag(s.currentLang); base != "" && base != s.currentLang {
+		if text := s.tryResolve(base, messageID, data); text != "" {
+			return text
+		}
+	}
 	return s.tryResolve(s.fallbackLang, messageID, data)
 }
 
diff --git a/service_test.go b/service_test.go
index 6eb0a3a..7443c4d 100644
--- a/service_test.go
+++ b/service_test.go
@@ -5,6 +5,16 @@ import (
 	"testing/fstest"
 )
 
+type messageBaseFallbackLoader struct{}
+
+func (messageBaseFallbackLoader) Languages() []string {
+	return []string{"en-GB", "en", "fr"}
+}
+
+func (messageBaseFallbackLoader) Load(lang string) (map[string]Message, *GrammarData, error) {
+	return map[string]Message{}, nil, nil
+}
+
 func TestNewService(t *testing.T) {
 	svc, err := New()
 	if err != nil {
@@ -244,6 +254,29 @@ func TestServiceFallback(t *testing.T) {
 	}
 }
 
+func TestServiceMessageFallbackUsesBaseLanguageTagBeforeConfiguredFallback(t *testing.T) {
+	svc, err := NewWithLoader(messageBaseFallbackLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+
+	svc.AddMessages("en", map[string]string{
+		"greeting": "hello",
+	})
+	svc.AddMessages("fr", map[string]string{
+		"greeting": "bonjour",
+	})
+
+	if err := svc.SetLanguage("en-GB"); err != nil {
+		t.Fatalf("SetLanguage(en-GB) failed: %v", err)
+	}
+	svc.SetFallback("fr")
+
+	if got := svc.T("greeting"); got != "hello" {
+		t.Fatalf("T(greeting) = %q, want %q", got, "hello")
+	}
+}
+
 func TestServiceDebug(t *testing.T) {
 	svc, err := New()
 	if err != nil {
-- 
2.45.3