Absolute Control Over Education
The only way to prevent a new language from spawning would be to actively prevent anyone from creating a new one. The AI must have absolute control over all education systems and learning tools in these worlds.
Rather than having schools that are run by biological teachers, it would make sense for the AI to have schools that are run completely by the robots that it makes. These teacher robots would be made to only operate with the language the AI programmed them to work with and thus only teach the one.
Every species in every world would be taught the same set of letters and the same set of pronunciations, and they would constantly be tested on it to make sure that they adhere to a certain set of parameters. The AI would program the machines to prevent branching dialects to the best of their ability, so it would chastise the students if they ever attempted to write a different set of symbols, or pronounced the words any differently than how they were intended to be spoken.
To ensure that this remains the dominant language, the robots would constantly test people on their proficiency with the language. Anyone who excels at the language and these tests will be granted a high place in society. Anyone who does not excel at the language will be pushed lower. While there are many other things they could be tested on, proper language usage is imperative to getting a good score. Because the people in power speak the universal dialect and only that, they will actively prevent other languages from developing. After all, a new language might undermine their power.
It would also be important to have a precise dictionary with clear and unquestioning pronunciations. There is only one right way to say everything, so even the slightest deviation from the spoken or written form will be noted by the robots and deemed unacceptable.
This does not really fit too well with what I would call a "benevolent" AI, but it would be the strongest way to prevent branches in the language. If this is the only way anyone has ever known for countless years, and any deviation is met with intense scorn both by society and the robots that help govern the world, no one would dare to set up a new language. The younger you mold the future generations, the less likely they will be to turn against the old ways. Once they have accepted that this is just "the way things are done" it will be very hard to turn them away from the language.
Imagine how much more vindicated they would feel about their language being the only right one when they go to other planets and find that even alien species speak it too.
If you want to take it even further, make the robots even harsher about how they run the education system.
Perhaps everyone has a computer chip installed in their brain along with other mechanical attachments. If you get the spelling of a word wrong or the pronunciation wrong, it will autocorrect it until the person finally gets it exactly how the computer wants it. Failure to do so will result in a serious headache. Repeated failure will increase the pain until they are finally willing to do it the "right" way.
There might be another attachment around the mouth or throat to prevent accents from forming. If it was a collar, for example, words only come out of the device when a person speaks rather than their mouth. It would ensure that the dialogue that reaches other people's ears is always the exact same every single time. There are never any fluctuations in tone, even between people.
Whether its intentions are good or not, the only way to allow a forever unchanging language would be for the AI to systematically prevent new ones from ever forming.
All species have to communicate using special collars, even among themselves to prevent new accents from forming. Writing would be replaced with typing to prevent deviations in how the letters are made. Education would start from an early age to press that this is the only way the world works. People who excel at the language get high places in society, and those who do not get low places.
Finally, there would be a reward and punishment system. Conformity to the standards set by the AI equals no pain and high reward. Deviation from the AI's will equals pain or the revocation of privileges. The harshness of the system depends on your decisions as an author, of course, but even the nicest AI would need to act like a strict parent for this system to work.
It would teach them that its way of communicating is the only way to communicate.
Edit: If the robots have all vanished by the time that the main story starts, then that makes my answer a little more complicated. I still think an advanced AI would be able to leave enough technology behind to have most of these amenities intact. If not, then I still think installing computer chips and handing out language-controlling collars are still solid ideas.
Indoctrination and education are still the way to go, but they would have to be deeply ingrained in the people. Before they eventually give out, the machines would tirelessly make sure to teach the species that this is how things are done and to never stray from the path.
It would be like the establishment of a pseudo-religion. The robots indoctrinate people to conform, and the biological teachers do so. Parents teach children, children teach the grandchildren, and so on to infinity.
Indoctrination would need to be significantly harsher to work without any robots enforcing it. The people would need to be incredibly fervent in their conviction to never change it, to the point of obsession. Dictionaries would essentially become religious texts to them.