Probably eventually they can, given the increasing computing power and storage available on phones. Google is a cloud company though, and don't control the hardware (other than the tiny segment of market share Pixel has) so I wouldn't look for them to do that.
Some might suggest Apple is the more likely candidate, with their emphasis on privacy and an SoC twice as fast as anyone else's. However, even with all the cloud resources Google is able to throw at it, Google Translate is still laughably bad for trying to act as a Babel Fish, cute pre-planned demo notwithstanding. If you doubt that, pick a random Reg sized news story written in another language and let Google have a go at it, and see how poorly it does. It does well enough to kind of get the gist of it, but it hurts your brain to read, and would drive you nuts if you had it speaking that way in your ear!
Even if Apple could match that poor translation ability, which they probably can't, running on a phone instead of in the cloud, which may not be possible, it wouldn't be nearly good enough.