The scope of this not well defined:

• Are all message groups in scope, or only translatable pages (and maybe message bundles)? Asking because it’s the result of the translatable pages research.
• In case of translatable pages, when should the notification be delivered: when the page is edited or when the page is marked for translation? Probably the latter, as watching the source page (cf. T257748) is already a solution for the former.
• Which categories should there be notifications about? Should users be able to toggle individual categories?
  • New translation unit
  • Removal of a translation unit
  • Change to a translation unit with fuzzying
  • Change to a translation unit without fuzzying
  • Manual fuzzying, without source message change
  • (On translatable pages) Change to a translation unit where only tvar content changes and translations are not fuzzied (these cause no warnings in Tux and require no updates in most languages, but could require updates e.g. in languages with vowel harmony)
  • (On translatable pages) Change to the translatable page template (i.e. anything outside of the <translate> tags)

All message groups. We can discuss what triggers a notification as we make further progress on the subtasks. We might start with the simplest case of when there are new messages to translate.

Thanks for the clarification! Gradual introduction of triggers is a good idea. However, I think the most important trigger is fuzzying translation units (primarily through changes, secondarily manually), as definitions sometimes change substantially (without being completely rewritten), leaving fuzzy translations misleading, and these should be fixed ASAP, while missing translations fall back to other languages, which makes them harder to understand, but not easier to misunderstand.