diff --git a/service.go b/service.go
index 28426be..a650d3c 100644
--- a/service.go
+++ b/service.go
@@ -791,14 +791,25 @@ func (s *Service) getMessage(lang, key string) (Message, bool) {
 
 // AddMessages adds messages for a language at runtime.
 func (s *Service) AddMessages(lang string, messages map[string]string) {
+	lang = normalizeLanguageTag(lang)
+	if lang == "" {
+		return
+	}
+
 	s.mu.Lock()
-	defer s.mu.Unlock()
 	if s.messages[lang] == nil {
 		s.messages[lang] = make(map[string]Message)
 	}
 	for key, text := range messages {
 		s.messages[lang][key] = Message{Text: text}
 	}
+	tag := language.Make(lang)
+	if !slices.Contains(s.availableLangs, tag) {
+		s.availableLangs = append(s.availableLangs, tag)
+	}
+	s.mu.Unlock()
+
+	s.autoDetectLanguage()
 }
 
 // AddLoader loads translations from an additional Loader, merging messages
diff --git a/service_test.go b/service_test.go
index dd7ca79..fca3ae4 100644
--- a/service_test.go
+++ b/service_test.go
@@ -677,6 +677,25 @@ func TestServiceAddMessages(t *testing.T) {
 	}
 }
 
+func TestServiceAddMessages_RegistersLanguage(t *testing.T) {
+	svc, err := New()
+	if err != nil {
+		t.Fatalf("New() failed: %v", err)
+	}
+
+	svc.AddMessages("fr_CA", map[string]string{
+		"custom.greeting": "Salut!",
+	})
+
+	if err := svc.SetLanguage("fr_CA"); err != nil {
+		t.Fatalf("SetLanguage(fr_CA) failed: %v", err)
+	}
+
+	if got := svc.T("custom.greeting"); got != "Salut!" {
+		t.Fatalf("T(custom.greeting) = %q, want %q", got, "Salut!")
+	}
+}
+
 func TestServiceHandlers(t *testing.T) {
 	svc, err := New()
 	if err != nil {