LanguageSpecificFileRule.cs

public class LanguageSpecificFileRule : RegExRewriteRule
{
	private string _defaultLanguageIso2;

	public override void Initialize(UrlRewritingNet.Configuration.RewriteSettings rewriteSettings)
	{
		base.Initialize(rewriteSettings);

		_defaultLanguageIso2 = rewriteSettings.GetAttribute("defaultLanguage", "de").ToLowerInvariant();

		if (Rewrite != RewriteOption.Domain)
			throw new InvalidOperationException("RewriteOption must be Domain!");
	}

	public override bool IsRewrite(string requestUrl)
	{
		var isRewrite = base.IsRewrite(requestUrl);
		return isRewrite;
	}

	public override string RewriteUrl(string url)
	{
		var langIso2 = UriUtils.GetSubdomain(new Uri(url));
		var relativeUrl = new Uri(url).AbsolutePath;
		var filePathWithLang = GetFilePathWithLang(relativeUrl, langIso2);
		var filePathDefault = GetFilePathWithLang(relativeUrl, _defaultLanguageIso2);
		var languageSpecificPath = GetLanguageSpecificPhysicalPath(filePathWithLang);

		return File.Exists(languageSpecificPath)
				? filePathWithLang
				: filePathDefault;
	}

	private string GetFilePathWithLang(string url, string langIso2)
	{
		var dotIndex = url.LastIndexOf('.');
		return url.Insert(dotIndex, "-" + langIso2);
	}

	private string GetLanguageSpecificPhysicalPath(string fileNameWithLang)
	{
		return HttpContext.Current.Server.MapPath(fileNameWithLang);
	}
}