I think communicating your expectations before starting is a must and keeping the lines of communication open. If they're expecting that they can have friends to stay whenever for weeks at a time at your expense and you're not okay with that then they need to know that now!
Make sure they've had a chance to read through the contract well before arriving so they can ask someone in their own country to explain things if necessary. I think it also reassures parents to see the more formal arrangements like pay, holiday and notice written out.
Have a manual with every possible bit of information written in it. It's invaluable even when you speak English fluently to have step by step instructions for the complicated washing machine or want to know what a child finds comforting, such as a special toy or story. If you don't speak the language then having something written to refer back to becomes all the more important.
Food - they should be welcome to eat as you would expect your own teen/young adult to eat, which doesn't necessarily mean what they want when they want if you want them to be a good, healthy model fir the children. I'd also advise coming up with a system to clearly mark food which is out of bounds to them.
Visitors - it's up to you again what you feel comfortae with. Most people say no overnight guests unless you've met them/they're family and only for 1 or 2 nights at a time.
Phone - investigate a calling plan which includes their home country. Providing a cheap duty mobile is a good idea as it also ensures they're contactable. Tell them you don't want to hear the house phone ringing all day and all night, that it's your house phone too so they shouldn't tie up thd line and when they're on duty they're not to be on the phone, unless babysitting and the children are in bed. Most use skype these days anyway so encourage that if they have wifi access.