How do I change language of my make theme?

Hello.

I downloaded the .mo file for Swedish and created a ‘languages’ folder in wp-content (this is what I read on other forums). Then I changed define(‘WPLANG’, ”); to define(‘WPLANG’, ‘sv_SE’); in wp-config and my whole site crashed. I tried everything to get it back, even contacted my hosting service. Then I said fuck it and deleted everything in order to start fresh.

I did the whole process again and the site crashed. So now I know what not to do. How do I do this the right way?