The extension is intended to make it easy to set Firefox's default search engine to Startpage, which we think it does pretty well.

Unless we're misreading you, what you're suggesting can be achieved by setting French as the preferred in language in Firefox (about:preferences#general) and doing the same in Startpage settings (https://www.startpage.com/do/settings).

If you want to discuss it further, and perhaps clarify your feature request, do reach out at support@startpage.com.