Translation procedure fail

Hello,

I have few problems with translation on my webpage.

Croatian wordpress installation is used from the start. After instalation, Metro Magazine theme was uploaded. Theme provides .po file to so user could translate the theme.
Inside wp-config.php file define('WPLANG', 'hr'); was set. But as instructed for the theme I translated theme .po file and named it hr_HR.po file and changed the line in wp-config.php file to define('WPLANG', 'hr_HR');
That hr_HR.po is theme translation file that theme developer gave me with the theme that I bought.
After that change, default Croatian translation, for example comments section went back on English language.
Some theme parts/words/sentences were translated and some not because theme creator did some mistake while making translation file.

My questions:
Why after making change in wp-config.php file my wordpress went back to English version? Is my procedure of using theme .po file wrong? How to leave Croatian .po file inside wp-config.php file and still make that theme .po file work? I’m not sure that my whole procedure is done correctly. How to make that both files work in the same time?

Also this theme have widgets that also have their own translation files. For example I use famous WP Polls widget that also have its own translation file. After I translated front end words and named file wp-polls-hr_HR.po nothing happened. What is wrong in this procedure?

Please help me if you can, my deadline is March 1st and this are last bugs that I have to solve.

Thank you very much in front for you all assistance and all the best!
Miroslav

Link to website is:
https://www.samoborinfo.hr