Great question — answering as the UphillTech team:
Pinterest is using the subdomain method of internationalization . Let's take a look: tr.example.com <-- Subdomain Method Despite Google saying that they rank subdomains more or less the same as subdirectories/folders , SEOs still debate it. Strong opinions on this likely come from success after implementing it, which most likely has nothing to do with the method itself, but rather how it's implemented and the unique site it's being implemented on. Subdomains are considered standalone sites and distinct from the main (root) domain. A subdomain enjoys the benefits from being part of the same site in terms of reputation, crawl rate, spam score, and tens of if not a hundred other ranking...
Full answer by Mike Ciffone: https://webmasters.stackexchange.com/a/137069 (CC BY-SA 4.0).
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