- Employees. Employees (not independent contractors), don't require any paperwork. The employer acquires copyright in all creations completed within the scope of employment.
- Independent Contractors. Independent contractors would be bound by work made for hire provisions in their contracts. That's acceptable because a "translation" is one of the acceptable work made for hire categories.
- Volunteers. Volunteers do not receive a payment. However, that doesn't mean that some consideration is not passing to these folks. "Consideration" is the answer to the question as to why someone is performing a task. And as long as you can point to some consideration for volunteers -- for example, interns acquiring work experience, translators acquiring job references or a portfolio -- you can use a work for hire agreement with the volunteers. You can state "for consideration, the sufficiency of which is acknowledged," in the agreement (although if both parties sign, that's not necessary (scroll down) in a majority of states).
Can you make a pre-work assignment? Technically, the translator has no copyright to transfer until the work is finished or "fixed." That's when the copyright first manifests itself. Some agreements bypass this issue by having the person promise to assign the work and to grant a power of attorney for such purposes. For example, pre-invention assignment agreements typically include copyrights and these agreements have routinely been found to be valid.