Internationalization: Not working "post" method with set_locale package?

Hi all.
I want to be my website works in 2 locales (en , ko)
so I followed this tutorial
I installed set_locala package
and it will look like this.

in router.ex

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
    plug SetLocale, gettext: TextingWeb.Gettext, default_locale: "ko", cookie_key: "project_locale"
  end

scope "/:locale", TextingWeb do
    pipe_through :browser # Use the default browser stack

    get "/", PageController, :index

    # Sign in and out
    get "/sign-in", AuthController, :new, as: :sign_in
    post "/sign-in", AuthController, :new, as: :sign_in
    get "/sign-out", AuthController, :delete, as: :sign_out
    get "/sign-in/:token", AuthController, :create, as: :sign_in
    get "/sign-up", UserController, :new,  as: :sign_up
    post "/sign-up", UserController, :create, as: :sign_up

    get "/confirm-email", ConfirmEmailController, :new
    post "/confirm-email", ConfirmEmailController, :create

  end

It works ok, It displays other locale (Korean), but when I tried to sign-in, it doesn’t work.
params that I submited from sign-in form ignored,

[debug] Processing with TextingWeb.AuthController.new/2
  Parameters: %{"locale" => "ko"}
  Pipelines: [:browser]

It only returns %{“locale” => “ko”}
how can I work around this?