Working with many resource files (Resources.resx) in ASP.NET C#

I'm implementing a multilanguage ASP.NET MVC 5 site and everything is working fine. I have a helper to check the language of the user and all my controllers extends from my own BaseController to check what language must use.

I don't know exactly how it works, and now, I would like to add more than one resource file to have more "confortable" and organized my strings... for example, I would like to have Resourse-Login.resx, and Resource-Login.es.resx, etc.

I have read this official tutorial (link), but I can't find information about how really it's working.

In the tutorial I have read, one phrase called me my attention:

Now right click on "Resources" project and then choose "Add->New Item" context menu command. Choose "Resource File" and name it "Resources.resx". This will be our default culture (en-US) since it has no special endings.

The special endings make the "magic"? How it doing the ResourceManager to know how itself must work?

If this part of my code a good practice?

// Modify current thread's cultures            
Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo(cultureName);
Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;

I paste the code I'm using:

Helper:

public static class CultureHelper
{
    // Valid cultures
    private static readonly List<string> _validCultures = new List<string> { "af", "af-ZA", "sq", "sq-AL", "gsw-FR", "am-ET", "ar", "ar-DZ", "ar-BH", "ar-EG", "ar-IQ", "ar-JO", "ar-KW", "ar-LB", "ar-LY", "ar-MA", "ar-OM", "ar-QA", "ar-SA", "ar-SY", "ar-TN", "ar-AE", "ar-YE", "hy", "hy-AM", "as-IN", "az", "az-Cyrl-AZ", "az-Latn-AZ", "ba-RU", "eu", "eu-ES", "be", "be-BY", "bn-BD", "bn-IN", "bs-Cyrl-BA", "bs-Latn-BA", "br-FR", "bg", "bg-BG", "ca", "ca-ES", "zh-HK", "zh-MO", "zh-CN", "zh-Hans", "zh-SG", "zh-TW", "zh-Hant", "co-FR", "hr", "hr-HR", "hr-BA", "cs", "cs-CZ", "da", "da-DK", "prs-AF", "div", "div-MV", "nl", "nl-BE", "nl-NL", "en", "en-AU", "en-BZ", "en-CA", "en-029", "en-IN", "en-IE", "en-JM", "en-MY", "en-NZ", "en-PH", "en-SG", "en-ZA", "en-TT", "en-GB", "en-US", "en-ZW", "et", "et-EE", "fo", "fo-FO", "fil-PH", "fi", "fi-FI", "fr", "fr-BE", "fr-CA", "fr-FR", "fr-LU", "fr-MC", "fr-CH", "fy-NL", "gl", "gl-ES", "ka", "ka-GE", "de", "de-AT", "de-DE", "de-LI", "de-LU", "de-CH", "el", "el-GR", "kl-GL", "gu", "gu-IN", "ha-Latn-NG", "he", "he-IL", "hi", "hi-IN", "hu", "hu-HU", "is", "is-IS", "ig-NG", "id", "id-ID", "iu-Latn-CA", "iu-Cans-CA", "ga-IE", "xh-ZA", "zu-ZA", "it", "it-IT", "it-CH", "ja", "ja-JP", "kn", "kn-IN", "kk", "kk-KZ", "km-KH", "qut-GT", "rw-RW", "sw", "sw-KE", "kok", "kok-IN", "ko", "ko-KR", "ky", "ky-KG", "lo-LA", "lv", "lv-LV", "lt", "lt-LT", "wee-DE", "lb-LU", "mk", "mk-MK", "ms", "ms-BN", "ms-MY", "ml-IN", "mt-MT", "mi-NZ", "arn-CL", "mr", "mr-IN", "moh-CA", "mn", "mn-MN", "mn-Mong-CN", "ne-NP", "no", "nb-NO", "nn-NO", "oc-FR", "or-IN", "ps-AF", "fa", "fa-IR", "pl", "pl-PL", "pt", "pt-BR", "pt-PT", "pa", "pa-IN", "quz-BO", "quz-EC", "quz-PE", "ro", "ro-RO", "rm-CH", "ru", "ru-RU", "smn-FI", "smj-NO", "smj-SE", "se-FI", "se-NO", "se-SE", "sms-FI", "sma-NO", "sma-SE", "sa", "sa-IN", "sr", "sr-Cyrl-BA", "sr-Cyrl-SP", "sr-Latn-BA", "sr-Latn-SP", "nso-ZA", "tn-ZA", "si-LK", "sk", "sk-SK", "sl", "sl-SI", "es", "es-AR", "es-BO", "es-CL", "es-CO", "es-CR", "es-DO", "es-EC", "es-SV", "es-GT", "es-HN", "es-MX", "es-NI", "es-PA", "es-PY", "es-PE", "es-PR", "es-ES", "es-US", "es-UY", "es-VE", "sv", "sv-FI", "sv-SE", "syr", "syr-SY", "tg-Cyrl-TJ", "tzm-Latn-DZ", "ta", "ta-IN", "tt", "tt-RU", "te", "te-IN", "th", "th-TH", "bo-CN", "tr", "tr-TR", "tk-TM", "ug-CN", "uk", "uk-UA", "wen-DE", "ur", "ur-PK", "uz", "uz-Cyrl-UZ", "uz-Latn-UZ", "vi", "vi-VN", "cy-GB", "wo-SN", "sah-RU", "ii-CN", "yo-NG" };
    // Include ONLY cultures you are implementing
    private static readonly List<string> _cultures = new List<string> {
    "en",  // first culture is the DEFAULT
    "es" // Spanish NEUTRAL culture


};
    /// <summary>
    /// Returns true if the language is a right-to-left language. Otherwise, false.
    /// </summary>
    public static bool IsRighToLeft()
    {
        return System.Threading.Thread.CurrentThread.CurrentCulture.TextInfo.IsRightToLeft;

    }
    /// <summary>
    /// Returns a valid culture name based on "name" parameter. If "name" is not valid, it returns the default culture "en-US"
    /// </summary>
    /// <param name="name" />Culture's name (e.g. en-US)</param>
    public static string GetImplementedCulture(string name)
    {
        // make sure it's not null
        if (string.IsNullOrEmpty(name))
            return GetDefaultCulture(); // return Default culture
        // make sure it is a valid culture first
        if (_validCultures.Where(c => c.Equals(name, StringComparison.InvariantCultureIgnoreCase)).Count() == 0)
            return GetDefaultCulture(); // return Default culture if it is invalid
        // if it is implemented, accept it
        if (_cultures.Where(c => c.Equals(name, StringComparison.InvariantCultureIgnoreCase)).Count() > 0)
            return name; // accept it
        // Find a close match. For example, if you have "en-US" defined and the user requests "en-GB", 
        // the function will return closes match that is "en-US" because at least the language is the same (ie English)  
        var n = GetNeutralCulture(name);
        foreach (var c in _cultures)
            if (c.StartsWith(n))
                return c;
        // else 
        // It is not implemented
        return GetDefaultCulture(); // return Default culture as no match found
    }
    /// <summary>
    /// Returns default culture name which is the first name decalared (e.g. en-US)
    /// </summary>
    /// <returns></returns>
    public static string GetDefaultCulture()
    {
        return _cultures[0]; // return Default culture
    }
    public static string GetCurrentCulture()
    {
        return Thread.CurrentThread.CurrentCulture.Name;
    }
    public static string GetCurrentNeutralCulture()
    {
        return GetNeutralCulture(Thread.CurrentThread.CurrentCulture.Name);
    }
    public static string GetNeutralCulture(string name)
    {
        if (!name.Contains("-")) return name;

        return name.Split('-')[0]; // Read first part only. E.g. "en", "es"
    }
}

BaseController:

public class BaseController : Controller
{
    protected override IAsyncResult BeginExecuteCore(AsyncCallback callback, object state)
    {
        string cultureName = RouteData.Values["culture"] as string;

        // Attempt to read the culture cookie from Request
        if (cultureName == null)
            cultureName = Request.UserLanguages != null && Request.UserLanguages.Length > 0 ? Request.UserLanguages[0] : null; // obtain it from HTTP header AcceptLanguages

        // Validate culture name
        cultureName = CultureHelper.GetImplementedCulture(cultureName); // This is safe


        if (RouteData.Values["culture"] as string != cultureName)
        {

            // Force a valid culture in the URL
            RouteData.Values["culture"] = cultureName.ToLowerInvariant(); // lower case too

            // Redirect user
            Response.RedirectToRoute(RouteData.Values);
        }


        // Modify current thread's cultures            
        Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo(cultureName);
        Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;


        return base.BeginExecuteCore(callback, state);
    }
}

RouteConfig (use to add /en /es in the request to detect the language)

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
        routes.MapRoute(
            name: "Default",
            url: "{culture}/{controller}/{action}/{id}",
            defaults: new { culture = CultureHelper.GetDefaultCulture(), controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}