Oh this is a bad sign from the start!! Excuse my ''wtf?'' and NO, it is not ok to have bruises from 5yrs old all the time! It may happen 1 or 2 times (maybe an accident or the beginning of abusing...) but if the problem is that serious that you have to write it here about it then you should think about having a serious talk with the kid and the parents.
If it is not working, leave. Your mental health and self esteem are very important and is no point to work for a family that doesn't appreciate you.
Now, I know what you mean, been there, done that and I learned my lesson and what I have to do. You shouldn't let yourself kicked by a child, you are the adult, he is the child, he has to understand it is not right what he is doing to you, you have to show him you like him and you can be his friend but you have to show him that he has to respect you.
The langauge is a problem- ask the parents to help you to communicate with him. Either he should tell his parents, in front of you, why is he naughty or he can tell them, when you are not around, and then the parents to tell you what he said.
Together with the parents you should do this (for the translation so that things are easier for all of you, as they are English and German speaking):
First- you have to speak to him- why is he behaving like this? what is his problem? Listen to him (eventhough you don't understand that much, at least you are there in front of him), see what is his reason, tell him he hurts you (!) and try to find a way to solve it, together.
Second- you should talk with the parents about your own feelings, you are a human not a robot! Together with the parents, you have to come up with an efficient discipline method and you as adults should stick to the method!
Then you have to tell the parents that the next time he will hit you again, he will be grounded! Learn some basic words in German like telling him he will be grounded, he is grounded and why.
In my case, the method that I applied when I was a nanny and was efficient was to: 1. Have a talk with the child, listen to him, try to solve the problem. 2. Tell him he will be grounded if he is not behaving well. 3.If he is naughty again, tell him he is grounded, why is he grounded and take him to a separate room. Close the door and ask him to stay there and think about what he's done. When he is calm, he can come out. No toys or his favourite items should be around.
If this is not working and they don't want to solve the problem, it means they don't respect you, they don't appreciate you and you should find another family to work for.
Good luck.