A good translation conveys the meaning of the original accurately and fluently in the target language. Keep in mind that often there is no “exact match” for concepts from one language to another. Despite what many people think, a literal, word-for-word translation does not guarantee accuracy.
Rather than cram the original concepts into the target language, quality translations adapt the core message to the target audience. A free translation is not necessarily less accurate than a literal one.
It is also inadvisable to assume that a “certified” translator is going to be better than one who is not. As a general rule, it is much safer to assess a translator’s proficiency by his or her real performance rather than by a vague claim of certification or a test taken sometime in the past.
The ultimate judges of quality are your readers, who should be able to read the translated text as effortlessly as those who read the original. Attaining that result is the ultimate benchmark of a successful translation.