> https://github.com/uav4geo/LibreTranslate/issues/33

That issue is about emojis, which confused me a bit, but I see that the linked commit is about replacing <unk> by the corresponding source token. https://github.com/argosopentech/argos-translate/commit/6a0f...

That's definitely an improvement, but what I was actually wondering about was why the model was copying the input verbatim in the first place.

Looking through the en-zh data in Opus, it looks like a bit of a mixed bag. This sample from OpenSubtitles v1 contains an ad for MyDVDrip in the Chinese version only: http://opus.nlpl.eu/OpenSubtitles/v1/en-zh_sample.html Filtering the data with some heuristics might be a good idea.