UrlProvider for generating multilingual URLs in 1:1 multilingual Umbraco sites - see http://www.alexlindgren.com/archive/building-multilingual-sites-in-umbraco-with-vorto-part-2/

MultilingualUrlProvider.cs

using System;
using System.Collections.Generic;
using Umbraco.Web.Routing;

/// <summary>
/// MultilingualUrlProvider
/// </summary>
public class MultilingualUrlProvider : IUrlProvider
{

    public IEnumerable<string> GetOtherUrls(Umbraco.Web.UmbracoContext umbracoContext, int id, Uri current)
    {
        var originalRoute = umbracoContext.ContentCache.GetRouteById(id);
        var urls = new List<string>()
        {
            "/es" + originalRoute
        };
        return urls;
    }

    public string GetUrl(Umbraco.Web.UmbracoContext umbracoContext, int id, Uri current, UrlProviderMode mode)
    {
        
        var originalRoute = umbracoContext.ContentCache.GetRouteById(id);
        if (umbracoContext.PublishedContentRequest != null)
        {
            var culture = umbracoContext.PublishedContentRequest.Culture;
            return "/" + culture.Name + originalRoute;
        }
        else
        {
            return "/en-US" + originalRoute;
        }
    }
}

RegisterEvents.cs

using Umbraco.Core;
using Umbraco.Web.Routing;

public class RegisterEvents : ApplicationEventHandler
{
    protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        UrlProviderResolver.Current.InsertTypeBefore<DefaultUrlProvider, MultilingualUrlProvider>();
    }
}