In the Eye of the World, Aginor and Balthamel, both recently out of the Dark One's prison, confront Rand and co. fluent in common. Both should have only be able to speak the old tongue, and the book leads us to believe that these too have been out for days, maybe weeks, but that they also have been waiting at the eye of the world or near it for Moiraine and Rand to show up. How did they learn common so fast? Did Ishmael teach them or was it given to them from the Dark One himself.
Most of the Forsaken learned quickly because the Old Tongue and the current common tongue are closely related. Aginor and Balthamel were held close to the surface of the sealing of the Bore and were able to watch what was going on in the world while they were trapped, so they saw the evolution of the language and could speak it immediately.
See the answer for Week 14 here: Tor Questions of the Week
I do not remember which book this is in, or I'd be able to provide a more exact answer, but one of the scenes of the Forsaken talking to each other is told from the point of view of one of the Forsaken. In this scene, there is a very brief mention that the 'common' tongue was known as the 'vulgar' tongue before Shaitan's prison was breached. As such, they are already familiar enough with the tongue that they can speak and be understood, though they would have a strange accent unless they put effort into adjusting to newer dialects.
Edited to remove elements that were not adequately researched, and thus not appropriate in a post such as this. My apologies for going beyond the elements I recalled that were more directly relevant.