Hello Yanichka,
I understand your fears but, that is life, everyone is like that.
First, for the "loyalty" to your boss, that is not a problem at all, if he is that nice, he will be happy for you to do well and improve. I wouldn't think of that so much.
For the loneliness, if you are lonely in your place, so what are you going to loose by coming here?
From my experience, there are a lot of German students here, what branch of science you do? we have a complete lab of Germans in the school.
Second, when you arrive here, there are several ways to make friends:
1- go to live in a student hall, you can make a lot of friends there, everybody is in the same boat.
2-you can go to the trips organised by the university and meet people.
3-you can join sport societies or what ever society you like.
4-At the start a lot of students will be lonely and they just talk at the bus stop (that is how I became friend with someone)
5-your lab and school is also another possibility.
But, you have to make the friendship right at the start of the course, cause that is the time when every one is new to here and want to make friends.
if you search the website of the lab you want to join in, some of them show the nationality of the students currently in the lab, not every university do that but some do.
am saying that life at university is not like a work place, it is vibrant and full of other types of experiences, and that is part of coming abroad, it expands your mind and challenge your limitations, at the end you feel stronger and wiser, but it is very difficult at times and also quite depressing, but you have to know how much you want to do this.
YOU REALLY NEED TO KNOW HOW MUCH YOU WANT TO DO THE PhD, AND DO YOU THINK IT WORTH THE SACRIFICES OR NOT, IF YOU THINK IT IS LIFE LONG DREAM, THEN DO IT, AND FIGHT FOR IT, THAT WHAT MAKES IT NICE AT THE End. if YOU THINK YOU ONLY WANT IT FOR A JOB OR JUST TO GET OUT FROM YOUR PLACE, I DON'T THINK THEN IT IS A GOOD IDEA TO DO IT.
Lastly, it took me one year until someone replied to me and accepted to supervise, I have emailed at least 7 potential supervisors, some people get the offer from the first time, it depends on determination and luck.
Good luck