Subtitle won't change by translate

Hello, I am using “Flat theme” and have one problem using it with Polylang.
When I am editing a page, I have an option under the text editor like this:
https://i.imgur.com/HTgZU4c.png – so I can change this subtitle on the website: https://goo.gl/s4x9Ji

Problem is, that after I set the subtitle in main language, it’s automaticaly copying this main option to the second language too.
Same thing happen when I set it up for the second language (it’s copying to the main language). So polylang can’t separate these strings. Can you help me what to do? It’s really urgent for me and I can do anything for you, to help me solve this. Thanks ver much!

https://www.ads-software.com/plugins/polylang/