var supportedCultures = new[] { "en", "ru", "es" };
var localizationOptions = new RequestLocalizationOptions
{
    DefaultRequestCulture = new RequestCulture("en"),
    SupportedCultures = supportedCultures.Select(c => new CultureInfo(c)).ToList(),
    SupportedUICultures = supportedCultures.Select(c => new CultureInfo(c)).ToList()
};

// Провайдеры (порядок важен — первый найденный выигрывает)
localizationOptions.RequestCultureProviders.Insert(0, new QueryStringRequestCultureProvider());
localizationOptions.RequestCultureProviders.Insert(1, new CookieRequestCultureProvider());
localizationOptions.RequestCultureProviders.Insert(2, new AcceptLanguageHeaderRequestCultureProvider());

app.UseRequestLocalization(localizationOptions);