In response: while there are a TON of caveats and specific use case variances, I’d say, on the whole, a subdomain setup is my *least* favorite way to implement internationalization.
With a subfolder structure you get the benefits of the site’s existing authority, as you move into new regions (NOTE this assumes the domain is starting on a non- ccTLD domain. You can’t start on, say, a . domain and expect amazing results in other countries.)
With a ccTLD structure, you get the localization signal benefits, which may help for SEO - and also for users (it’ll depend on the region, but some regions *really* like a local domain, and some don’t care. Local customer preference matters! To determine this, a good rule of thumb is to Google some of the target terms in the target regions. are all ccTLDs for that region ranking? A mix? or mostly non-ccTLDs?)
A subfolder is a fine setup, structurally speaking, but it doesn’t give you either of the benefits of the above setups - so again, it tends, on the whole, to be my least recommended method (baring insights from other biz requirements, technical constraints, etc etc)
