A team of 50 humans visit Mars via a flying saucer. They discover canals and a city teeming with Martians. The Martians speak English, but how?
They are smart, and have been listening to our television and radio broadcasts for over a century. The Martians, in fact, are about a century ahead of us in electronic transmission of information. When we were first using a telegraph, they had already advanced beyond that to what is our "modern" communications infrastructure.
This includes highly compressed (using mathematical algorithms) data that, to us, always looked like just noise (the more compression you get, the more the message looks like a random string of 1s and 0s, because compression is the recognition and removal of structure in data, thereby leaving what looks like noise). (Further, no matter how fast your transmitting medium, even at the speed of light, transmitting less data is faster than transmitting more: So compression is an evergreen technology, always useful, and always better if it does more).
Plus our other infrastructure: hard-wire cables, fibre-optic, lasers, etc reduce the noise we are sending out into space considerably; we really don't have to send any noise at all if we were very concerned about that.
The Martians have chosen to minimize their own radio "leakage" and have stayed a century ahead of us in the necessary technology, so we have never noticed them. Until we arrived. But they have been monitoring OUR transmissions since we first began. That is how they know English.
A pretty standard SF workaround is that the Martians are telepathic, and to keep us from freaking out they arrange that we "hear" them in our own language. Which would really be the only way that concepts of any detail could be comfortably conveyed. It is not freaky at all when you hear a sound and realize that a nearby creature is making it, especially if the sound is words in a language you understand. Even if the creature is not moving its mouth, much.
Telepathic projections affecting visual input would be considered frightening hallucinations and it is hard to convey detailed concepts via smell, taste, or waves of raw emotion. Or waves of proprioceptive sensibility, if you are Thursby's proprioception people.
The truth is Martians don't speak English. In fact, they speak Martian or, at least, one of the Martian languages. The Martians have thought transference or a form of linguistic telepathy. When they speak their Martian language is translated into English or whatever language is spoken by the person they are talking to.
Since this takes place in an alternate reality where the tropes of nineteen-fifties science-fiction are its reality and this includes thought transference as a means to break the language barrier between humans and aliens.
The boring alternative answer is learning English had become the latest fashion trend. Since all Martians are super-smart it only took them a few days to become completely fluent in English.