Original GT-MT3000 web interface theme

Could someone point me out where the files for original web interface skin is located if it’s possible? I can edit luci themes for advanced web interface but I would like to know if it’s possible to edit/tweak original router web interface as well.

Also if it’s possible to add another language or to volunteer as a translator for another language I would love to participate in that.

P.S. I’ve found themes located in www folder, but can’t find the file where I can add another language menu item.