Aurebesh

A language was written on a poster that was on a wall in Mayor Me's trap street. (TV: Face the Raven)

Behind the scenes
Although not referred to in the episode, the language is Aurebesh, which originated from the Star Wars films. When translated it spells out "DeLorean", the time travelling vehicle for the Back to the Future franchise. The word is also next to an image of a device resembling the flux capacitor from that series.