I also think the situations where I'd find useful this kind of translations are when I'm not in my home country which most likely means my data plan does not work or is very expensive, which makes it less useful than it sounds at first.
Also, being able to travel around in europe without any extra roaming charges, means that I can still use this, wherever I go in europe.
I'm very excited to try these as soon as I can get my hands on them.
Many people may not have Fi, which I get, but I get the feeling Google's ideal use case is these headphones paired with a Pixel 2 running on Fi.