If such a thing would even be possible.
closed as primarily opinion-based by dot_Sp0T, sphennings, Frostfyre, Monica Cellio♦ Nov 12 '17 at 3:53
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All you need for this is proximity to English-speaking people. If there's proximity, there will be conflict, or potential conflict which can be avoided through communication.
Merfolk might be very territorial about their favorite fishing grounds. "You can fish here, but not there, landlubbers!"
Merfolk might have a habit of rescuing shipwrecked sailors. Over time they've learned to communicate with these flotsam-folk so as to not freak them out.
Could be they desire trade with the humans. They could trade shipwreck treasure, exotic corals, or heck, fish to the humans in exchange for metals and ceramics.
There could be a protection racket going on. "Nice ship you have there, human. Be a shame if something ... happened to it."
Might be that merfolk all know English, but nobody on land knew, because merfolk lore maintains that "the men up there don't like a lot of chatter"...
I see several.
Merfolk are magically transformed (english speaking) humans.
Merfolk could have no useful language of their own, and have discovered english, and have adopted it. The same could be true of clothing, makeup, and Viagra.
English speaking humans captured pet merfolk, taught them english, and then released them, much as dolphins in Day of the Dolphin were taught certain sounds.
I think it is more of a problem to have verbal language underwater in the sense you are asking for an air-driven mechanism to exist, where air is absent. But you don't seem to be asking about that very basic problem. So, they likely have english, and viagra, and boob jobs too. Why not.
If you want sailors to jump overboard when they hear you, it helps if the sailors understand what you are singing.
Ulysses and the Sirens. https://commons.wikimedia.org/wiki/Herbert_James_Draper
If any one unwarily draws in too close and hears the singing of the Sirens, his wife and children will never welcome him home again, for they sit in a green field and warble him to death with the sweetness of their song. There is a great heap of dead men's bones lying all around, with the flesh still rotting off them. Therefore pass these Sirens by, and stop your men's ears with wax that none of them may hear; but if you like you can listen yourself, for you may get the men to bind you as you stand upright on a cross-piece half way up the mast, and they must lash the rope's ends to the mast itself, that you may have the pleasure of listening. If you beg and pray the men to unloose you, then they must bind you faster.
Look at Ulysses. He definitely hears them. I wonder if later in his life he ever dreamt of that song and woke up as he was walking down the road to go to them.