Whilst contracts may not be obligatory, you may wish to formalise any work with a contract which both you and the linguist agree upon. This may be especially pertinent if the job is a large one such as the translation of a book.

Alternatively you may wish to just agree to the work specification via email. It is up to you.

You may wish to see a lawyer about drafting a contract, or you could use a contract supplied by the linguist, or you could even draft your own. An example of the type of things which can be found in contracts can be seen by clicking here. Please note that this is just an example and we are not responsible in any way for it's legality. If you are in any doubt about drafting a contract you should seek the advice of a lawyer.