First, I assume that the base language of the site is Polish and that English is one of the alternate languages.
In order to change the text of the navigation for people whose language is English, you first have to change your language to English. As you mentioned, you have two different accounts, one set to Polish the other to English, so use the English one. Confirm that the UI is is English by seeing that terms like “Share”, “Edit”, and “Search this site” are in English now, not Polish.
Click on “edit” next to the navigation menu, then on the “…” and “Edit” next to the menu item to be translated. Replace the original Polish Display name “O nas” with English Display name “About”. Do not change the URL, that is not language-specific. Repeat for all menu items, then press the Save button. If the editing prompts, for example the “Save” button were in English and the site language was not, then the text that you changed will only affect the English version.