Somewhat related but not he same:
While that's a good idea, I think it should also be possible to specify on the subscriber list (like https://meta.wikimedia.org/wiki/Global_message_delivery/Targets/Tech_ambassadors) in what language you want to receive the MassMessage. Preferably with fallback support, so as an example one could enter "pt, pt-br, simple, en".
Use case examples:
- https://www.wikidata.org/wiki/Wikidata:Warung_Kopi#Tech_News:_2020-21 was available in Indonesian but was delivered in English because Wikidata is English I guess.
- Delivery on multilingual projects is always in English. As an example (one of many), https://www.mediawiki.org/wiki/User_talk:Kghbln?uselang=de is a German user, but the newsletter is delivered in English. Because MediaWiki is English I guess.
- When Simple English is available, this will sometimes (but not always) be preferable over English if the preferred language is not available, but not always. The reverse can also happen: someone who mostly works on Simple English Wikipedia and wants a newsletter delivered there may very well prefer the regular English version. Just because you edit Simple English doesn't mean you don't understand regular English.
- If a user from the US happens to know German and is mostly active on German Wikipedia to edit articles about the US, they may want newsletters delivered to their user talk on dewiki... in English. So you can't even blindly trust the language code of the wiki.