There are interesting reports in Private Eye (unfortunately, the web site does not seem to include the details) about the contracts for interpreters.
It seems that the job of interpreter is being deprofessionalised and there are many compromises with quality as a result of saving money. One shocking case is where Czech interpretation is being used for Slovak users whereas in the Czech Republic, Slovak suspects are given interpreters using Slovak. As well as the danger of misinterpretation of nuance, there are the general affective factors of suspects being helped by people who do not share the same language.