If you know of a good source for large piles of docs that have been accurately/naturally translated eng/ch/jp, I'm sure deepl would be interested. As another poster pointed out, any deep learning NLP project boils down to quantity+quality of data. I'm assuming adversarial approaches don't work well in this context but I'm not very familiar with nlp research.

Right but that's kind of the essence of machine translation. You won't always have high quality parallel training data for all languages so you have to find a way to thrive with low quality data.

Google has clearly made it their mission to solve that problem, and I'd say they've been rather successful.