First, if the alien race has mastered intergalactic travel then they most likely have translation technology… its an earpiece or a technological implant that automatically translates your language into their language, and when they speak, the auto-translator speaks in English.
Alien species for sure have databases of languages spoken around the universe, and they exchange these files with each other much like we exchange music and pictures.
For Christ’s sake.. google is developing this now!! They aliens freaking have it!
A more advanced form of technology is super-learning like Neo did in the Matrix. The alien will download the dictionary or words and language patterns into his brain on the spot, it is then able to speak whatever language it chooses. It is still debatable whether it would take for that download process to happen or if it will be instantaneous.
Vocal cords and pronunciation might be an issue as well, so the alien might have a computer device, much like an EEG device or an implant that translates the alien’s thoughts into spoken language by a spoken computer.
And last but not least, aliens will probably use and in most cases have used TELEPATHY.
Telepathy is really the universal way of speaking between all species. Contactees and abductees who have reported experiencing telepathy with aliens, reported it as a strong LINK.. mind to mind communication, very much to a fear performed by Charles Xavier from Xmen.
The recipient.. the human being.. was very powerless when it came to preventing his mind from being read, all the human needed to do was to think out loud and the alien would understand him.