HI

To generate the resource files automatically, follow these steps:

1. In Solution Explorer, double-click the Default.aspx file.
   
   Note The Default.aspx file opens in Design view.
2. On the Tools menu, click Generate Local Resources.
   
   Note By default, a new folder that is named App_LocalResources is created. Additionally, a resource file that is named Default.aspx.resx is created. This file contains information about each Label control on the page. The values in the resource file match the values that you entered for each Label control in Design view.
3. In Solution Explorer, right-click the Default.aspx.resx file, and then click Copy.
4. In Solution Explorer, right-click the App_LocalResources folder, and then click Paste.
5. In Solution Explorer, right-click the Copy of Default.aspx.resx file, and then click Rename.
6. Type Default.aspx.es-mx.resx, and then press ENTER.

For more details please read:

http://support.microsoft.com/kb/917414

Hope it helpful.