For me, it's all about semantic domain strengthening, not subdomains so I run the entire multilingual structure on subdirectories: /es, /de, /fr, and so on. In my opinion, only really big, truly massive projects should go the subdomain route, because your overall development budget basically multiplies by the number of language versions you're running
Like I said before, I'm using directories, not subdomains, for the extra language versions. English is the main one in practice everything else is secondary. No duplicate pages here, 'cause it's all about solid technical seo proper hreflang tags that flip depending on the language version you're on. Bing and Google? Yeah, there's a difference in multilingual ranking. Googles way bigger in terms of geo coverage and just gets more usage overall. Technically though, both search engines work the same way. Content wise obviously, it's gotta be unique for each region, with its own local flavor. And yeah, link building runs across every single language version