If you don’t hire a professional, your documentary exhibits may not be admitted and your witnesses may be unable to testify. You need to have a court approved translation or interpreter.
That is how important it is. This applies to immigration filings as well.
This answer applies to documents that will be used in court proceedings or to court interpreting. If you have other documents that are to be used in private institutions, such as a college transcript for example, and you need it translated to be admitted into a post-graduate program, that is a different story. Every school sets its own requirements. I specialize in “Legal” translation and interpreting and can only speak about that.
As to what requirements your legal translation company should meet, I suggest you look at my response to another question here.
So, why do I need a LEGAL translation professional? Notice the emphasis on “legal”.
There are many companies that offer translations and interpreting, but very few of the large ones focus on the United States legal system.
What I have noticed is that the vast majority of them have a plethora of services being offered to the website visitor. Just go to any translation service website and see how they try to offer literary translations, marketing and website translation, conference interpreting and healthcare interpreting, to name a few. Translating and interpreting methods vary based on the translation type. A legal document is translated very differently than would a manual or a novel.
Also, these other types of translations are much more profitable from the business side of things. Meaning that a translation company will make a lot more money translating a manual for a corporation, compared to translating a few emails you need as evidence, and they may put off your tasks and give priority to their bigger, corporate clients.
I suggest you look for a company that specializes in legal translation and interpreting and not just offers the service “on the side”.