I just got a used MacBook Pro laptop, the problem is that it's in French and I would like it to be in English. Is there a way to install a legal copy of the OS in English if I have a legal French copy?

Open System Preferences and click International. Drag the language you want to use to the top of the list. (If you don't see the language, click Edit.) Select a text behavior for the script appropriate for your language. To select the keyboard and input methods you use, click the Keyboard Menu tab.

You can also display dates, times, numbers, and currency according to the conventions of your region.

To use a keyboard or input method, choose it from the Keyboard menu (the one with a flag).

And restart