I have been using MessagEase for a pretty long while now and it was a great choice for polyglots like me because I could handle all my 11 languages on one keyboard. Unfortunately Thumbkey doesn't have that on the fly flexibility yet. Options like ad hoc adding letters and a compose key and combining diacritics are vital for me. Happy to help to implement this, otherwise I might even fork and rework if I find the energy...
When I use Emacs, it's with Evil.