We call them additional officials or fifth or sixth man. Tonight I watched the Champions League match between and Ajax and Dinamo Kiev. The reporter was looking for a better name for the extra officials.
Any suggestions? Or do you already have a specific name for them in English?

We like the term “AAR” for Additional Assistant Refereee.
@joeyref would call it: “waste of space and waste of money would be my suggestion :)”, as he said on Twitter. Bit long name for me 😉