WordPress / Theme localization

Hello,

I’ve been using WordPress for a while now, but just recently turned my attention to localization issues/problems as it wasn’t asked of me before.

I have my WordPress site installed in English (en_US), so out of the box. The theme i’m running has a .po file but no .mo file (and can’t find any reference to localization in the theme functions.php.

Now, my question is how can i just add another language to WordPress so that front-end ‘strings’ appear in a language, but the admin stays in English?

On top of this, how would i go about making wordpress make use of the .po file from my theme?

Just to be sure i’m clear: i do not want a plugin to display my content in two-three-four…etc languages //// or install WordPress directly in my language.

I just need front-end to be in one language and back-end remain in English.

PS: I know i could just go about translating every string in the code to the language i want, but then i run an update to my installation, all that would go away.

Thank you in advance to anyone who answers and regards,
George