From bae8b1b788e827a76f735a224969dcedcd707ed8 Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Thu, 2 Apr 2026 05:31:48 +0000
Subject: [PATCH] fix(i18n): fall back to base fallback language tags

Co-Authored-By: Virgil <virgil@lethean.io>
---
 service.go      |  8 +++++++-
 service_test.go | 20 ++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/service.go b/service.go
index bac71c6..8fd85a3 100644
--- a/service.go
+++ b/service.go
@@ -392,7 +392,13 @@ func (s *Service) resolveDirect(messageID string, data any) string {
 			return text
 		}
 	}
-	return s.tryResolve(s.fallbackLang, messageID, data)
+	if text := s.tryResolve(s.fallbackLang, messageID, data); text != "" {
+		return text
+	}
+	if base := baseLanguageTag(s.fallbackLang); base != "" && base != s.fallbackLang {
+		return s.tryResolve(base, messageID, data)
+	}
+	return ""
 }
 
 func (s *Service) resolveWithFallback(messageID string, data any) string {
diff --git a/service_test.go b/service_test.go
index 7f7a8ad..16f69b4 100644
--- a/service_test.go
+++ b/service_test.go
@@ -296,6 +296,26 @@ func TestServiceMessageFallbackUsesBaseLanguageTagBeforeConfiguredFallback(t *te
 	}
 }
 
+func TestServiceMessageFallbackUsesConfiguredFallbackBaseLanguageTag(t *testing.T) {
+	svc, err := NewWithLoader(messageBaseFallbackLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+
+	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-CA")
+
+	if got := svc.T("greeting"); got != "bonjour" {
+		t.Fatalf("T(greeting) = %q, want %q", got, "bonjour")
+	}
+}
+
 func TestServiceCommonFallbackUsesBaseLanguageTagBeforeConfiguredFallback(t *testing.T) {
 	svc, err := NewWithLoader(messageBaseFallbackLoader{})
 	if err != nil {
-- 
2.45.3