The first recommendation is simple: always look for a professional translator or interpreter. A person who speaks more than one language, in spite of being able to speak it fluently, may not be able to translate at a professional level. Grammar, syntax and context are very important when translating, and someone who does not have experience may have a difficult time translating correctly. It may even take more time, as they may not have the range of vocabulary that a professional does.
If you are going to hire a freelance translator, make sure he/she has studies in the field or is at least certified. Years of experience are also a good indicator. To make sure that the translation is of high quality, the company or translator should offer you a proofread translation (yes, by more than one person) for better quality control.
If you want to hire a company, check their online reviews. Their previous clients will serve as a reference by sharing their experiences, so you will have an initial reference point. You can also ask for samples of completed work.
Request a quote for your work. This way you will know if the service is within your budget. Keep in mind that if the company offers very low prices, you should be careful, because quality has a price. It is better to pay a little more for a good result from the beginning than to pay double if you get a poorly finished outcome.
Value the treatment and attention the company offers you; this also speaks about the company’s professionalism. If you do not obtain the assessment you want, it is very possible that it may not be the adequate option for you. If you have doubts about the quality of the work delivered, it is best to ask for a revision by a professional. A second opinion is always preferable to having a flawed translation.