Yes, it can be useful to sign a contract with the contributors to make clear that:
- They offer their services free of charge
- They give you permission to use their translation in your project
- They won't suddenly retract their permission, no matter what you do with your project (change it, sell it, decide to monetize it in some way, etc.)
- They take liability for damages caused by their work, for example when the translation contains inappropriate language and it gets your game censored from the app store.
This is of course a wish-list from your perspective. Individual volunteers might want to negotiate some of these points. For example, I would be very wary of accepting a liability clause for work I don't get paid for.
It can be useful to get professional help for drafting the language of these contracts.