According to the most recent census, English is not the main language of 4.2 million people in England and Wales (7.7 per cent of the population); 726,000 people cannot speak it well and 138,000 speak no English at all. Many of us non-native speakers will at some point have to deal with the justice system, in one capacity or another (my first exposure was as a juror). The right to be tried in a language you understand is guaranteed under Article 6 of the European Convention on Human Rights (ECHR).

Translation is like rubbish collection: no one notices it until something goes wrong. The need to move from community-based interpreting in the courts to a regulated system became apparent in the early 1990s, after a number of miscarriages of justice. Iqbal Begum was tried in 1981 for killing her abusive husband; poor interpretation meant she didn’t understand the difference between murder and manslaughter. Released on appeal in 1985, she committed suicide a few years later. Her case led to calls for an independent regulatory body, and in 1994, following a recommendation by the Royal Commission on Criminal Justice, the National Register of Public Service Interpreters (NRPSI) was established.

In 2011, when I joined the NRPSI, registered interpreters were called directly by the courts and paid £85 for the first three hours, with a lower rate afterwards, plus travel costs. The arrangement was thought too costly, and in 2012 the Ministry of Justice outsourced its court interpreting services. A £90 million contract was awarded to a small agency, which was promptly bought by the outsourcing giant Capita.

Capita began using unqualified interpreters, paying them £16 per hour (qualified interpreters were eventually upgraded to £22), with no minimum time and lower travel fees. Many interpreters boycotted the agreement for financial or ethical reasons. There were protests outside Parliament, letters to MPs and 'dossiers of shame' detailing instances of unprofessionalism. A Justice Committee report in 2013 called the new system 'nothing short of shambolic'.

Capita’s contract expired last year. The service provider until 2020 is thebigword. Early signs are worrying. On day one, a 'slave gang' trial was adjourned because the interpreter who had worked on it could no longer afford to attend. Eight weeks on, a man who tried to steal £600 worth of groceries was held in custody for 48 hours before an interpreter could be found. The contract differentiates between 'complex' and 'standard' jobs; the latter include plea and bail hearings, can be assigned to anyone with 'basic interpreting qualifications' (a 60-hour course booked through Debonair Languages, one of the subcontractors used by thebigword, costs £25), and are paid an hourly rate of £18, with no compensation for the first hour of travel.

The MoJ claims to have saved £48 million on court interpreting since 2012. This figure doesn’t include the cost of delays: 2661 cases were adjourned between 2011 and 2015 because there was no interpreter available, but the costs were not recorded. It wasn't until the end of 2015 that Capita was able to reach its attendance target, supplying a court interpreter 98 per cent of the time. The case against Colonel Kumar Lama, a Nepalese army officer accused of torture, finally collapsed last year, having cost £1 million. His trial was adjourned in 2015 because they couldn’t find interpreters.

Much of court interpreting is simultaneous: you sit next to a defendant and whisper in their ear as you listen to the proceedings. You have to be familiar with legal procedures and fluent in legalese as there is no time to decode 'ABH' or invent a term for 'corporate manslaughter'. You also need to be able to temper your language depending on who you are interpreting for: a drug addict going through withdrawal, a graduate with some knowledge of legal arguments, or an emotionally unstable person with a patchy understanding of the situation. These skills require constant practice (I keep my hand in by doing the occasional hearing when booked directly by the prosecution, and that's barely enough).

An EU directive requires interpreters to deliver a 'quality sufficient to safeguard the fairness of the proceedings'. Mistakes, from linguistic inaccuracies to cultural subtleties, cannot be entirely avoided; but some blunders are striking: a man charged with perverting the course of justice called a ‘pervert’; ‘bitten’ confused with ‘beaten’; ‘charge’ translated as ‘fine’ rather than ‘accusation’. Some people provided with an interpreter are grateful for any help they can get; others are past caring. A defendant once asked me just to sit quietly as all the interpreters he had seen were useless.

Interpreters are struggling to present a united front: some have given up on court work until the outsourcing agreement is scrapped, others brandish their diplomas and blame novices for undercutting them. In the first four weeks of the current contract, only 123 of 12,010 assignments were classified as complex enough to require an interpreter who met the pre-2012 standards.

The Language Shop is the MoJ contractor responsible for 'independent quality assurance'. It has been looking for assessors to conduct random checks ('mystery shopping') in courtrooms, at fees only slightly above those offered by thebigword. One of the Language Shop’s rival bidders was the NRPSI, which has since lowered its admission criteria, adding to its list of recognised qualifications a diploma issued by the International School of Linguists, thebigword's 'training partner'. Unlike the old-school qualifications, this is based on a distance course with no exams.

As qualified interpreters stop working for the courts, standards keep slipping – yet more evidence, if it were needed, that outsourcing doesn't improve services. (Whether the administration of justice should be considered a service, and interpreting an overhead, is another question.) Technological advances may one day make human interpreters obsolete, but the robots can't yet deal with such formulations as 'attempted battery, or the threat thereof'. Meanwhile, the government is keen for every UK resident to learn English (preferably at their own expense).

‘The obligation of the competent authorities is not limited to the appointment of an interpreter,’ the ECHR says, ‘but … may also extend to a degree of subsequent control over the adequacy of the interpretation provided.’ When asked what will happen to the right to interpretation and translation in courts after Britain leaves the EU, the MoJ said it ‘will not be providing a running commentary’.