How to change language

I'm using Ata 3.4.4. and WP 2.8.6

I've managed to switch my dashboard to my language by downloading the appropriate WP language file to wp-content/languages/ and by setting the language in wp-config.php, but I seem unable to switch the theme language.

I've downloaded and unzipped the .po and .mo files and uploaded them to
wp-content/themes/atahualpa/
Anything else I need to do besides that?

How can I tell if the translation has been successful? Some parts are obviously still in English. Yes, I know some parts must be edited via the theme options, but how can I be 100% sure that what is supposed to be translated really is? E.g. it still says "One comment" if the post has one comment. Shouldn't that have changed?