Site language change by update of .po & .mo files

Hi there,
I have bought a website theme and have set it up on a server in wordpress. This theme is in English, but fully translation ready.
I want to change theme labels to different language (polish).
In WordPress installation I have changed language to Polish in General Settings.

Now, I have never used wordpress before this project and I’m not a programmer and my knowledge is very limited – just for the record ??

I have done some research and found out that in order to translate labels on the website I need to update .po and .mo files. I have purchased Poedit software and updated both of those files.

Then I have logged into web FTP on my server and uploaded/replaced pl_PL.po and pl_PL.mo in: “wordpress/wp-content/languages” folder.

Then, based on instructions I have found online, I wanted to update wp-config.php file and define language as:
define (‘WPLANG’, ‘pl_PL’);

Now, there are 2 things:
1. I do not have this line in my wp-config.php file
2. I read somewhere that in new wordpress version “WPLANG is deprecated in 4.0”

Can someone please help me figure out how to change it and connect/point my new pl_PL.po file to my website so it’s properly translated?

Someone game me advise that since there is an option to change Language in General Settings of WordPress that should take care of the translation but it’s not working.

Please help!