diff --git a/backend/internal/server/handlers_auth.go b/backend/internal/server/handlers_auth.go
index 4d23524..09c62b3 100644
--- a/backend/internal/server/handlers_auth.go
+++ b/backend/internal/server/handlers_auth.go
@@ -46,6 +46,15 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
+	// TEMP DEBUG (revert before merge): trace the language_code Telegram actually sent and
+	// what we seeded, to settle the "ru interface" question on the test contour.
+	s.log.Info("telegram auth debug",
+		zap.String("external_id", req.ExternalID),
+		zap.String("language_code", req.LanguageCode),
+		zap.String("username", req.Username),
+		zap.String("start_param", req.StartParam),
+		zap.Bool("created", created),
+		zap.Strings("variant_preferences", acc.VariantPreferences))
 	if created {
 		// First registration: re-evaluate moderated-chat write access, so a user who
 		// joined the chat before registering is granted on the spot (no chat_member