That is, they are able to translate (in all directions) novel languages that were not previously heard[0]. It is an open question, with likely a negative answers, that there is a universal grammar even among humans[1] (the definition itself is vague but even the most abstract version is suspect and highly likely to not be universal across species). I think no one will be surprised if it is always impossible to interpret an entire language based on only a few words (let alone do it in real time)
This isn't a knock down, because even a trained device is insanely useful, it's just a note about limitations and triage. This is awesome stuff and I can't wait for the day we have transnational headphones. It's an incredibly complex problem that I'm sure is not short of surprises.
[0] There are a few exceptions such as Star Trek TNG's episode Darmok, S5E2, where the Tamarians' language is unable to be translated due to its reliance on cultural references (the literal words are translated but the semantic meanings are not). It's a well known episode and if you hear anyone saying "Shaka, when the walls fell" (translates to "Failure") they are referencing this episode (often not using the language accurately but who cares (nerds. The answer is nerds)).