MVCPostall/Postall/Controllers/LanguageController.cs

using Microsoft.AspNetCore.Mvc;
using System.Globalization;

namespace Postall.Controllers
{
    public class LanguageController : Controller
    {
        public ActionResult ChangeLanguage(string lang)
        {
            if (!string.IsNullOrEmpty(lang))
            {
                Thread.CurrentThread.CurrentCulture = new CultureInfo(lang);
                Thread.CurrentThread.CurrentUICulture = new CultureInfo(lang);
            }

            Response.Cookies.Append("Language", lang, new CookieOptions { Expires = DateTimeOffset.UtcNow.AddYears(1) });

            return Redirect(Request.Headers["Referer"].ToString());
        }
    }
}