ASP .NET Core default language is always English

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();