how to make your own language ?

I have problem, Im admin of one site and thay asked me as a administrator to make zen cart on our language(serbian) and i said ok, so i need to ask you:
Can someone tell me which *.php files i need to translate and where are thay in our database (everything is setup as startup - we didnt edit anything yet, exept we install some blue theme - from this site - ) ??

Your best bet - download a few language packs and see how it's done.

http://www.zen-cart.com/index.php?main_page=index&cPath=40_46

If you speak English well, you can see

includes/languages/english
includes/languages/english.php

and see what you have to do. Just fill all the values for these defines for your own language.

When you're done, please consider donating your work back to Zen Cart so that other Serbian speakers can benefit.

There is a new language pack in the Free Software Add Ons area for Serbian.

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=1345