Problem translating WP 4.0.1

Hy, I have a problem.

I just made an e-commerce site based on wp, and I want to transalte it.
I have no problem with the content translation, but i want to translate the the theme and the plugin (for example: the add to cart button);

I found that an easy way for this is with Code styling localization plugin, I have done everything correctly but it’s not working, and what I have noticed is that my wp-config.php doesn’t have define”(‘wplang’ , ”); line, and if I add it manualy, it causes error and the site doesn’t open.

Can anyone help? Thank you