I agree that we definitely shouldn't auto-translate Travosin words into English in the documents. That would take away far too much from the game. I do, however, think that a large improvement for the game would be the dictionary idea you talked about. A couple of suggestions I have for it would be, most importantly, to have the hover tooltip of your translation guess be a toggleable option. For someone who wants to learn Trevosan better, having the hover pop up could spoil the experience a bit if they end up accidentally hovering over a word or start to rely on it too heavily. If it is a toggleable option that will allow you to turn it off and be forced to cross-reference your own notes more.
Additionally, I am not sure if you plan to have the dictionary lock in words like how portraits do; however, I would recommend one of two things: either have them simply serve as a more organized version of the current notes page with no confirmation, or only lock them in if you have 5 or so right. I know 5 seems like a lot, but I believe that a lot of the fun of the game is making guesses for what the words mean and then continually refining them as you get more examples of the words being used. If they are locked in too easily, then that part is taken away.