Language as filtering criteria
A few months ago I was working on a delivery audit for a client who sends mail in multiple languages. We discovered that the language of an email has a significant delivery impact. The same email in different languages was delivered differently, particularly at Gmail. Emails in a language I don’t normally receive email in were delivered to my bulk folder.
Other folks have commented on similar things. Some filters really do look at preferred language of the recipient and treat mail in other languages as problematic. I don’t think that’s unreasonable. I do get a lot of foreign language spam and there’s no real way to stop it. Many countries don’t require opt-out links, and so there isn’t a clear way to even unsubscribe.
Writing in the recipient’s local language is one way to minimize inappropriate blocking, even when you have permission to send mail.