add

routers/user/auth.go

@@ -549,7 +549,9 @@ func handleSignInFull(ctx *context.Context, u *models.User, remember bool, obeyR
 		}
 	} else {
 		// Language setting of the user use the one previously set
-		u.Language = ctx.GetCookie("lang")
+		if len(ctx.GetCookie("lang")) != 0 {
+			u.Language = ctx.GetCookie("lang")
+		}
 	}
 
 	ctx.SetCookie("lang", u.Language, nil, setting.AppSubURL, setting.SessionConfig.Domain, setting.SessionConfig.Secure, true)