On my site, I want to create a French-language section with pages aimed at a French audience. For SEO purposes, I intend to put all these French pages inside a /fr/ folder so all their URLs start with mydomain.com/fr/.

Can I implement a language switcher dropdown that will go to the equivalent page in the selected language? For example if the visitor is on the English book page and they switch to French they land on the French book page - which is a different page, not just a translation.

Anything else I should consider before I start structuring this new silo?

I can reply in more detail later, but this is a fairly common request.

You can use javascript to detect the browser language of the person viewing, so you could create a little HTML widget with just javascript in it to detect the browser's language, if it doesn't match the current language, display a modal/box on the page to offer them to go to the correct page.