if I purchase an iMac with a French operating system, can it be changed to English?

Mac OS can easily change the language. To learn how please click https://support.apple.com/guide/mac-help/change-the-system-language-mh26684/mac

And if the need arises for a French keyboard, I'm sure you can find an inexpensive French language keyboard while you are in the French speaking country. You could use any USB keyboard, they are inexpensive.

The English and French keyboards are different. Do you plan to purchase an English language keyboard for compatibility when you switch the language to English?