Solved by vdw in post #2 
              
              
                Rare words or out-of-vocabulary words are a fundamental challenge for NMT. You still find very recent academic papers addressing this. 
For example, for a very simple NMT task, I used an off-the-shelf NER system to replace, say, person names. So 2 sentences “I met Alice” and “I met Bob” would be con…