As the question states, do the Borg speak English, or is it a universal translator interpretation? Has this ever been addressed on screen? The Borg obviously have a few million cultures and languages to choose from.
It's never been addressed, but given the Borg's information dense, hive-mind intelligence, if they can recognize a species, they will likely communicate using that language. It would make the absorption of the new species easier. However, given their incredible breadth of conquered technologies, a universal translator should be easy enough to incorporate into their communication technologies.
Samples of such investigations of a species can be seen from early Borg actions in the Alpha Quadrant. Memory Alpha's Borg entry recounts:
By the 2350s, rumors of an alien race called "The Borg" had reached the Alpha Quadrant, inspiring exobiologists Magnus and Erin Hansen to set out in search of them. Their research took them all the way to the Delta Quadrant before they and their daughter Annika were assimilated in 2356. (VOY: "The Gift", "The Raven", "Dark Frontier")
Borg activity in the Alpha Quadrant, including the assimilation of the USS Tombaugh in 2362 and assimilation of outposts along the Romulan Neutral Zone in 2364, were complete mysteries to Starfleet. (VOY: "Infinite Regress"; TNG: "The Neutral Zone")
- My suspicion is the Borg do not randomly absorb species without investigating them for technological developments first. If a species has something they want, the probably absorb smaller elements of the species until they can learn information which will aid them in assimilation.
- On the rare occasions that the Borg were willing to open a dialog with individuals, they chose a single drone to speak for the Collective. Jean-Luc Picard was assimilated and given the name Locutus in the misguided assumption that such a representative would lower the Federation's resistance to assimilation. (TNG: "The Best of Both Worlds")
See Also: Why aren't we all Borg?
To the best of my knowledge it has never been addressed.
We do know that the Borg had encounters with a range of Alpha Quadrant races including the Ferengi, the Cardassians, the Romulans and the (human) Hansen family prior to encountering the Enterprise-D in TNG.
Based on what we know about the Borg, it's highly likely that by the events of "TNG: Q-Who?" they would be communicating in plain English to avoid any possibility of being misunderstood.