Eunice here from Shopify, thanks for posting, and welcome to the Community!
The 'Buy It Now' button language will change when you select a supported language through your theme, as these languages have pre-defined translations. Shopify themes support the following languages:
If you're not using a free theme from Shopify, then your theme was made by a third-party theme developer. To change the language of your third-party theme you can view the support documentation for your theme, or contact the theme developer. If you're unable to find documentation that supports your request, or, if you aren't able to contact the theme developer, I recommend keeping an eye on this thread for further responses. We have some excellent Shopify Partners and Experts active in the Shopify Community that may be able to help.
You can try to find the text "Buy it now" on the "en.default.json" or "fr.default.json". If you can not find that text then that's because your theme using the hard card.
If you can not do that, you can add me as a staff and I'll check it for you (my email: firstname.lastname@example.org)
Hope this helps!