Won't need to be much of a lip-reader or 'interpreter' if,as Elvis says, Terry is claiming he said six words (Oi, I never callled you a..) before the racist word he admits saying but the complainant says there was just the racist word . It's a matter of counting the utterances!
But his counsel is talking of the interpretation of one word. The expert evidence is of use in corroborating the complaint. Like all expert evidence it can be challenged; it's not sacred. It looks to be in the category of a lot of testimony of handwriting experts; it has limits on its value, to put it mildly.
If it's being tried by a District Judge, a trained barrister or solicitor of some years standing before he took the job, he understands the principles of evidence and the use of expert evidence. That may not bode well for Mr Terry.