Localization issue – possibly php problem

Hi.

How to combine installed language .po file with theme .po file?
When I changed inside wp-config.php file line:
define('WPLANG', 'hr'); – installed Croatian wordpress without custom theme
to:
define('WPLANG', 'hr_HR'); – theme .po file used
I lost all Croatian translation that comes with installation and I get translation from the theme.
If I revert that, I get oposite.

How to use both files? What to do to keep Croatian translation that came installed and to use in the same time .po file that came with the theme?

Any experience from other users that translated their wordpress site to local language and then also implement translation of new theme and plugins?

Thanks,
Miroslav