I prefer to join on public bounty since it's transparant for us, rather than private bounty. But if you are quality poster, try to join bitcoin pay bounty, since it's better than altcoin bounty right now
I also have this issue at the beginning when I start accepting private translation request, which is not posted on Bitcointalk and the employee just sent me a PM through Telegram. I always ask them to pay me first unless they're reputable projects with verifiable identity and so on. You should use your own brain and do your own DD if you accept this king of work.
On the other hand, a private job like this usually gives better salary than your usual social media bounty.