Localization in ASP.NET Core

The default providers are:

QueryStringRequestCultureProvider
CookieRequestCultureProvider
AcceptLanguageHeaderRequestCultureProvider

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

In Startup.cs put this fro Slovenian:

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

    services.Configure<RequestLocalizationOptions>(options =>
    {
        options.DefaultRequestCulture = new RequestCulture("sl");
        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();