Translation of new sentence with new words

I tried the demo of “eng-fra” translation.
I want to implement this algorithm on another area.
My question is what should I do when I try to translate new English sentence when it has new word which doesn’t exist in the original English sentences?

This is a hard problem for NLP when a new word is not in your vocabulary. Take a look at subword method (like SentencePiece). It may be helpful. Some pretrained embedding may also help.

here is an example.