This is edging into contract territory. Which may sound scary but contracts are just a set of rules everyone agrees to and then ignores until something goes wrong. It's the "if something goes wrong" part that probably worries you?
At minimum you should clearly write out in your message what the translators are allowed to do with your work. For example, only published on their 1 site, no ebooks or printed copies, etc. Basically, "here's what you can do. Everything else is off limits without my written permission." Do it in writing, leave a paper trail, verbal agreements don't last.