How to change the language

As I create a Polish-language website that needs subpages, and I found your plugin to be suitable for my needs, I tried to use it, but unfortunately some of the things visible to the end user are in English what looks unprofessional. I buried a bit in language files adding Polish, finally I registered here to add Polish language. In the end, he may be useful to someone.

However, I have a problem – how to introduce a new language to the plugin? Unfortunately, I have no idea how to do it.
I really care about your help
Thank you in advance.

oh, I would forget, despite the high level in understanding English, I’m unfortunately in his writing, I used google translate, so I’m sorry for englandoo