fix(i18n): fail empty loader registrations

Co-Authored-By: Virgil <virgil@lethean.io>

service.go

@@ -831,6 +831,14 @@ func (s *Service) AddLoader(loader Loader) error {
 		return log.E("Service.AddLoader", "nil loader", nil)
 	}
 	langs := loader.Languages()
+	if len(langs) == 0 {
+		if el, ok := loader.(interface{ LanguagesErr() error }); ok {
+			if langErr := el.LanguagesErr(); langErr != nil {
+				return log.E("Service.AddLoader", "read locales directory", langErr)
+			}
+		}
+		return log.E("Service.AddLoader", "no languages available from loader", nil)
+	}
 	for _, lang := range langs {
 		messages, grammar, err := loader.Load(lang)
 		if err != nil {

service_test.go

@@ -1033,6 +1033,18 @@ func TestServiceAddLoader_Nil(t *testing.T) {
 	}
 }
 
+func TestServiceAddLoader_NoLanguages(t *testing.T) {
+	svc, err := New()
+	if err != nil {
+		t.Fatalf("New() failed: %v", err)
+	}
+
+	empty := fstest.MapFS{}
+	if err := svc.AddLoader(NewFSLoader(empty, "missing")); err == nil {
+		t.Error("AddLoader() should fail when no languages are available")
+	}
+}
+
 func TestPackageLevelAddLoader(t *testing.T) {
 	svc, err := New()
 	if err != nil {