How to use non-english langage

Hello,
I've put Polish language (pl_PL.mo and pl_PL.po) into wp-content/themes/atahualpa351/ but I have no idea, how to activate them

Please, plesae. It couldn't be so complicaded.


P.S.
I don't know is it important - I have WP3 with Polish language pack:
continents-cities-pl_PL.mo
continents-cities-pl_PL.po
ms-pl_PL.mo
ms-pl_PL.po
pl_PL.mo
pl_PL.php
pl_PL.po

I've changed "atahualpa.pot" and I still have english texts.
Kind of magic
my test site

I beleive there is a setting in the wp-config.php you need to set to usw multi languages. Try a search at wordpress.org

I've changed files:
atahualpa.pot
comments.php
comments-paged.php
on Polish lang. and still have English texts.

Could you tell me in which file can I find signed on attached JPG?

Is it good idea to change on PL "legacy.comments.php"?

The most important question, is Atahualpa 3.5.1 compatible with WordPress 3.0?


Adam