In most cases, it is much better to assign portions of the document to translators rather than letting them work in a crowdsourcing model. It's not very efficient as translators might jump from one part of the document to another, wasting time and not have a goal to work toward. The best way to do it is ask each translator how many words he/she can complete by the given deadline and assign sections of the document accordingly. If a translator falls behind, it is always possible to change the assignment during the project.
The "free-for-all" model might work when dealing with a very short document with extremely tight deadline for example or for a very large documents where splitting the document might take too much time or where translators might jump in and out of the project. These are not common scenarios.