diff --git a/service.go b/service.go
index 8aac8fd..7dc32b1 100644
--- a/service.go
+++ b/service.go
@@ -131,6 +131,7 @@ func NewWithLoader(loader Loader, opts ...Option) (*Service, error) {
 		if err != nil {
 			return nil, log.E("NewWithLoader", "load locale: "+lang, err)
 		}
+		lang = normalizeLanguageTag(lang)
 		s.messages[lang] = messages
 		if grammarDataHasContent(grammar) {
 			SetGrammarData(lang, grammar)
@@ -290,7 +291,11 @@ func (s *Service) SetMode(m Mode)           { s.mu.Lock(); s.mode = m; s.mu.Unlo
 func (s *Service) Mode() Mode               { s.mu.RLock(); defer s.mu.RUnlock(); return s.mode }
 func (s *Service) SetFormality(f Formality) { s.mu.Lock(); s.formality = f; s.mu.Unlock() }
 func (s *Service) Formality() Formality     { s.mu.RLock(); defer s.mu.RUnlock(); return s.formality }
-func (s *Service) SetFallback(lang string)  { s.mu.Lock(); s.fallbackLang = normalizeLanguageTag(lang); s.mu.Unlock() }
+func (s *Service) SetFallback(lang string) {
+	s.mu.Lock()
+	s.fallbackLang = normalizeLanguageTag(lang)
+	s.mu.Unlock()
+}
 func (s *Service) Fallback() string {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
@@ -829,6 +834,7 @@ func (s *Service) AddLoader(loader Loader) error {
 		if err != nil {
 			return log.E("Service.AddLoader", "load locale: "+lang, err)
 		}
+		lang = normalizeLanguageTag(lang)
 
 		s.mu.Lock()
 		if s.messages[lang] == nil {
diff --git a/service_test.go b/service_test.go
index 0d12494..685065e 100644
--- a/service_test.go
+++ b/service_test.go
@@ -33,6 +33,18 @@ func (h serviceMutatingHandler) Handle(key string, args []any, next func() strin
 	return "mutated"
 }
 
+type underscoreLangLoader struct{}
+
+func (underscoreLangLoader) Languages() []string {
+	return []string{"en_US"}
+}
+
+func (underscoreLangLoader) Load(lang string) (map[string]Message, *GrammarData, error) {
+	return map[string]Message{
+		"greeting": {Text: "hello"},
+	}, nil, nil
+}
+
 func TestNewService(t *testing.T) {
 	svc, err := New()
 	if err != nil {
@@ -148,6 +160,28 @@ func TestServiceTDirectKeys(t *testing.T) {
 	}
 }
 
+func TestNewWithLoaderNormalisesLanguageTags(t *testing.T) {
+	svc, err := NewWithLoader(underscoreLangLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+
+	langs := svc.AvailableLanguages()
+	if len(langs) != 1 || langs[0] != "en-US" {
+		t.Fatalf("AvailableLanguages() = %v, want [en-US]", langs)
+	}
+
+	if err := svc.SetLanguage("en_US"); err != nil {
+		t.Fatalf("SetLanguage(en_US) failed: %v", err)
+	}
+	if got, want := svc.Language(), "en-US"; got != want {
+		t.Fatalf("Language() after SetLanguage(en_US) = %q, want %q", got, want)
+	}
+	if got := svc.T("greeting"); got != "hello" {
+		t.Fatalf("T(greeting) after SetLanguage(en_US) = %q, want %q", got, "hello")
+	}
+}
+
 func TestServiceTPluralMessage(t *testing.T) {
 	svc, err := New()
 	if err != nil {