Why have you never been able to do it? I set up a full mail system years ago on a Xen/Linux VPS with stuff like Postfix, maildrop, Courier IMAP, a custom set of MySQL tables for aliases and such, and at one point migrated my TLS from CACert to LetsEncrypt. I enjoyed some aspects of the huge pain in the ass that all of that was, and having it work nicely was great. Spinning up a new email alias was easy and free, so I created a new one for damn near every site I interacted with, which later turned into a form of lock in having to continue running my server.

The continual server maintenance was a pain in the ass, requiring me to remember in substantial detail how it all worked so that I could appropriately integrate new things I had to learn like SPF and DMARC. I’m glad to have had some detailed sysadmin experience, but I was so glad in the end to finally migrate away from all that and just pay Fastmail instead.

I still have nearly the same flexibility with Fastmail and my custom domains, but they’re the ones that need to do all the maintenance. I can’t scale across unlimited domains for the same zero marginal cost, but I can make it work for a reasonable price with a few domains and scale arbitrarily within that. I’m sure there are other hosts out there that do a similarly good job, and Fastmail hasn’t been without its own troubles, but it’s been a net win for me.

I don’t recommend running your own server. I won’t do it again. I do recommend building an army of custom aliases all at your own custom domain(s).

but translation certainly isn’t [done on device]

Google Translate has downloadable language packs and runs on device, including offline in foreign countries without roaming data. Including the live video OCR and replacement.

That’s not to say that what you experienced isn’t creepy, but it’s not necessarily cloud-driven.