service.go

@@ -264,7 +264,7 @@ func (s *Service) SetLanguage(lang string) error {
 	matcher := language.NewMatcher(s.availableLangs)
 	bestMatch, bestIndex, confidence := matcher.Match(requestedLang)
 	if confidence == language.No {
-		return log.E("Service.SetLanguage", "unsupported language: "+lang, nil)
+		return log.E("Service.SetLanguage", "unsupported language: "+lang+" (available: "+joinAvailableLanguagesLocked(s.availableLangs)+")", nil)
 	}
 	if bestIndex >= 0 && bestIndex < len(s.availableLangs) {
 		s.currentLang = s.availableLangs[bestIndex].String()
@@ -336,6 +336,18 @@ func (s *Service) PluralCategory(n int) PluralCategory {
 	return GetPluralCategory(s.currentLang, n)
 }
 
+func joinAvailableLanguagesLocked(tags []language.Tag) string {
+	if len(tags) == 0 {
+		return ""
+	}
+	langs := make([]string, len(tags))
+	for i, tag := range tags {
+		langs[i] = tag.String()
+	}
+	slices.Sort(langs)
+	return strings.Join(langs, ", ")
+}
+
 func (s *Service) AddHandler(handlers ...KeyHandler) {
 	s.mu.Lock()
 	defer s.mu.Unlock()

service_test.go

@@ -1,6 +1,7 @@
 package i18n
 
 import (
+	"strings"
 	"testing"
 	"testing/fstest"
 	"time"
@@ -89,6 +90,27 @@ func TestServiceAvailableLanguagesSorted(t *testing.T) {
 	}
 }
 
+func TestServiceSetLanguageUnsupportedIncludesAvailableLanguages(t *testing.T) {
+	svc, err := NewWithLoader(messageBaseFallbackLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+
+	err = svc.SetLanguage("es")
+	if err == nil {
+		t.Fatal("SetLanguage(es) succeeded, want error")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "unsupported language: es") {
+		t.Fatalf("SetLanguage(es) error = %q, want unsupported language message", msg)
+	}
+	for _, want := range []string{"en", "en-GB", "fr"} {
+		if !strings.Contains(msg, want) {
+			t.Fatalf("SetLanguage(es) error = %q, want available language %q", msg, want)
+		}
+	}
+}
+
 func TestServiceT(t *testing.T) {
 	svc, err := New()
 	if err != nil {