ASP .NET Core default language is always English

If you are using Ajax call to set the culture and respond the back to the client with JSON result.

You have to set the cookie value corresponded to ASP.Net as below in browser level. otherwise, localizer is unable to find the correct culture with cookie value.

var date = new Date();
date.setTime(date.getTime() + (30*24*60*60*1000));
expires = "; expires=" + date.toUTCString();
document.cookie = '.AspNetCore.Culture' + "=c=" + (data.message || "ko-KR") + "|uic=" +(data.message || "ko-KR")  + expires + "; path=/";
window.location.reload();

You are setting "arabic" as DefaultRequestCulture but DefaultRequestCulture is used if none of the built-in providers can determine the request culture. The default providers are:

1. QueryStringRequestCultureProvider
2. CookieRequestCultureProvider
3. AcceptLanguageHeaderRequestCultureProvider

Most likely the culture is determined from the Accept-Language HTTP header that the browser is sending.

You have to remove the AcceptLanguageHeaderRequestCultureProvider in order to fallback to DefaultRequestCulture. To do that, we can overwrite the RequestCultureProviders list of RequestLocalizationOptions and use only the other two providers. In Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    CultureInfo[] supportedCultures = new[]
    {
        new CultureInfo("ar"),
        new CultureInfo("en")
    };

    services.Configure<RequestLocalizationOptions>(options =>
    {
        options.DefaultRequestCulture = new RequestCulture("ar");
        options.SupportedCultures = supportedCultures;
        options.SupportedUICultures = supportedCultures;
        options.RequestCultureProviders = new List<IRequestCultureProvider>
        {
            new QueryStringRequestCultureProvider(),
            new CookieRequestCultureProvider()
        };
    });
}

and in Configure method just use app.UseRequestLocalization(); before app.UseMvc();