ASP.NET Core Request Localization Options

Inside your public class Startup:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    //...

    app.Use((context, next) =>
    {
        //get client prefered language
        var userLangs = context.Request.Headers["Accept-Language"].ToString();
        var firstLang = userLangs.Split(',').FirstOrDefault();

        //set allowed alanguage
        var lang = "en"; //default
        switch (firstLang)
        {
            case "hy": //allowed
            case "ru": //allowed
                lang = firstLang; 
                break;
            default:
                //client language not supported
                lang = "en"; //use our default
                break;
        }

        //switch culture
        Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo(lang);
        Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;

        //save for later use
        context.Items["ClientLang"] = lang;
        context.Items["ClientCulture"] = Thread.CurrentThread.CurrentUICulture.Name;
        
        // Call the next delegate/middleware in the pipeline
        return next();
    });

    //... then goes app.UseMvc etc..

}

You can get Accept-Language header from the current Request and set default language. Your code should be something like this:

services.Configure<RequestLocalizationOptions>(options =>
{
    //...

    options.RequestCultureProviders.Insert(0, new CustomRequestCultureProvider(context =>
    {                    
       //...
       var userLangs = context.Request.Headers["Accept-Language"].ToString();
       var firstLang = userLangs.Split(',').FirstOrDefault();
       var defaultLang = string.IsNullOrEmpty(firstLang) ? "en" : firstLang;
       return Task.FromResult(new ProviderCultureResult(defaultLang, defaultLang));
   }));
});

Another way to get the Accept-Language header with a framework call (ASP.NET Core):

HttpContext.Request.GetTypedHeaders().AcceptLanguage

as an additional option, Here is a full solution: Handle culture in route (URL) via RequestCultureProviders

as an option as well, here is a simplified code for CustomRequestCultureProvider:

options.RequestCultureProviders.Insert(0, new CustomRequestCultureProvider(context =>
{
    var lang = context.Request.GetTypedHeaders().AcceptLanguage.FirstOrDefault()?.Value.Value ?? Constants.Languages.EN_US;

    return Task.FromResult(new ProviderCultureResult(lang, lang));

}));