Subfolder vs subdomain
A subfolder (site.com/fr/) ties a section to the main domain and its authority; a subdomain (fr.site.com) is treated as a largely independent site that must rebuild its own authority from a lower base.
In practice
For a blog, a shop, or a language version, the subfolder immediately benefits the new section from the domain’s history. Subdomains retain legitimate uses — applications, technically separate environments. This is also the principle behind the sidecar setup: publishing under the client’s domain, as a subfolder.