Let's say you go to the city abroad at a court session as a claimant, respondent or representative - and you need local judicial interpreter.
In many countries there is a special exclusive category of so-called "sworn interpreters". Sometimes they are called simply "judicial interpreters".
As a rule, the interpreter who is the employee of claimant’s or respondent’s company has no right to act as interpreter at trial as in this case objectivity of interpretation can't be guaranteed.
If English-speaking tourist or businessman is detained as the individual for administrative offenses and furthermore on suspicion in commission of any criminal offenses on the territory, say, Germany or France, certainly, he won’t be engaged in search of the interpreter. It will be done by local police officers or judicial authorities. But if you act as a claimant or respondent at trial in arbitration court or at court hearing in a civil action, you can try to invite and offer court the candidate of the independent sworn interpreter of your choice. The right of court to accept or reject such interpreter.
For the successful interpretation at court, besides good skills of oral consecutive interpretation, the interpreter needs knowledge of legal language of that branch of the right which judicial proceedings belong to. For example, civil, commercial or marine law are very different areas with the specific terms and legal nuances. In equal conditions it is always better to choose the interpreter who has already had experience of successful interpretation at similar trials. First of all, interpreter’s absolute understanding of the language which legal proceedings are conducted in is important, as well as ability to interpret quickly and correctly all questions, answers and legal statements of two parties. It is supposed that any certified "sworn interpreter" just by definition meets all these qualification requirements. But if in this certain country there is no legalized category of sworn or judicial interpreters, it is better to find out in advance, what formal requirements to interpreters are imposed by judicial authorities of this country to admit them to interpretation at judicial hearings, at poll of witnesses, etc.
Anyway, the first thing for the interpreter is to have document confirming his qualification (university diploma of interpreter or certificate on passing of corresponding exam) and - as the second important factor - (not for court, but for the customer of interpretation) if the interpreter has experience of professional interpretation at court.
Not depending on whether it will be official sworn interpreter or just strong interpreter who knows legal nuances, for successful interpretation at court it is desirable to acquaint the interpreter in advance at least in general with case papers and positions of the parties in judicial dispute.