Craft 3 Multi Site - Entries/categories/assets fields aren't translatable?
If we want different tutors assigned per site (country) to this course, we can't as an entries field (pulling from teachers section) is not translatable/multi site capable.
  
  …
  
  Same for images etc.


All relational field types (Assets, Categories, Entries, Tags, and Users) have a “Manage relations on a per-site basis” setting (under Advanced). If you check it, each site will get its own set of relations.




  But matrix fields aren't translatable either.


Same - Matrix fields have a “Manage blocks on a per-site basis” setting, right below the big Configuration setting.