I'm not saying he is. But as an English person who is used to hearing the Scottish accent - I can sometimes have a problem understanding what has just been said. Surely to a Portuguese translator, who has probably never encountered even a slight Glaswegian accent before - an accent is even more likely to increase the possibility of misunderstandings.
The interpreter is British, speaks quite well Portuguese and also French. I asked her if she had difficulty to understand Mrs Cameron, and she said she had none at all (before any session she has a chat with the witness outside of the court room, precisely to get accustomed to the eventual accent).
The difficult part is to hear properly a person of whom you only see the back at a certain distance. The lawyer or the judge asks a question. The interpreter turns her head towards the witness and translates the question, her mouth being at about 40 cm of the witness' ear. Then the witness answers, looking in front towards the Judge. The interpreter turns her head to listen better, she can see the mouth's movements, then she translates for the judge, looking in front of her.
Of what I heard from the witness and then from the interpreter (in the case of Mrs Cameron, much more from the interpreter) I catch a meaning that I sometimes write in French, don't ask me how !
You have to realize idiomatic expressions can't by definition be translated literally. There too there's the spirit and the letter, a reality some seem not to be aware of.