ITL was developed as a simple and elegant solution to enable easier communication between space ships and inside space ports. It is a fairly straightforward language with simple grammar and single letter based writing system. What makes ITL special, and uniquely suited for interstellar communication, is that the written script can be pronounced in three greatly different ways. The three ways to pronounce ITL are designed in a way to allow all the species of known space to speak in at least one of them. In theory, mastery of ITL requires the ability to understand all three form of pronunciation, which is one of the first things taught in language classes once learners have mastered the script, but even when people can only understand one of them they are still able to communicate through writing, as all three forms use the same letters.
Fluency in ITL is a requirement for almost all jobs in space and it’s the most common second language in most education systems, even before other local languages. In many frontier colonies with colonists from different countries of a planet, it has even replaced the traditional lingua franca of their homeworld, and for many spacers its the only language they know.
While all species are able to pronounce one of the forms of ILT, there is an uncountable range of various accents even within people of the same species. Some species have a harder time than others with understanding heavy accents, but in most cases it’s simply a matter of hearing the accent spoken for a few hours to fully understand it.
Not all species have a hearing range that can detect the full voice range of some other species. People traveling to systems where this is an issue for them when talking with the local population often wear hearing aids that shift their voices into a range they can hear. All personal communicators have the same feature and capture voice as it s spoken to play it back at a different frequency simultaneously. Better models are even able to amplify voices to the hearing range of other species and not just the species for which it was made. For visitors to other planets and stations, whose voice needs amplification to be fully audible to the locals, it is considered common courtesy to do so when possible, rather than to depend on them to fish out their own comms to understand what is being said to them.
The three forms of pronunciation are designed so that all species can pronounce one of them, though many are anatomically able to pronounce more than one. Talking to other species in the form they commonly can be an endearing party trick, but is almost never expected. Only one species has ever shown the ability to speak ITL in all three forms of pronunciation, but ironically they are the most isolationist species, that also uses very little verbal communication in general.