The neural networks are great at faking it, and they'll make up text that looks completely reasonable all day long. If that means inventing subjects and choosing random verb tenses, that's what they'll do. If you pick closely-related languages, you might be able to get a near perfect translation just by putting each word through a dictionary, and the neural networks can disambiguate things and smooth over the problems in the output by looking at local context. If you pick distantly-related languages, you might routinely need context from paragraphs away, or context which isn't present in the text at all but relies on some kind of human experience.