Hi Kasia,
I hope you are well today and thank you for your question.
You can try changing it by translating the theme in to your language “szukaj” as described on the following page.
The same with “No comments yet” (or can I remove it?),
Yes you can remove it by using the solution posted in the following topic.
https://colorlibsupport.com/t/remove-no-comments-yet/
Best Regards,
Movin