I live in California. My neighbours are very weird persons. They are from Russia. I don't know whether it is normal or not in Russia but in this family farther spank her daughter, who is about 10 or 11, with switch. And I could see several times that she is completely nude. Is this a case for police?

Or keep your sick fantasies to yourself.
Think about the consequences that this will bring to the family should you do this. I would just leave it alone unless I seriously felt that the child was in more danger there than the foster home full of strangers that they could be placed in. Foster care isn't always utilized but it is often enough to take it into consideration.
I am not saying that it is OK because they are Russians, I am just saying to take all factors you know into consideration before calling CPS.
Any time you move to another country you should research it's laws. If they didn't or whatever, that's their problem, not yours. Call the police.
What really concerns me is the child's nudity. Eleven is way too old to be naked for convenience/comfort, which means that she was stripped as part of the punishment. That adds sexuality and embarrassment to her physical punishment, which is frightening to me.
I would call social services. Hopefully they can coach the parents, which will give those kids a better life.
But our Grand and Great-Grandparents did use slender green limbs from trees as switches. You know, those ones that make the high-pitched whistling noise when you swung them around?
Yeah, that shit hurts.
The nakedness is what really makes me think this might not be completely normal. :C
I used to get smakced by my dad and after 5-10 smacks on my bare ass it didnt hurt anymore, so i was punished instead wth a belt and always bare so it hurt more.
I'm Ukrainian born and I was never punished like that, although my dad was spanked black and blue with a belt buckle on a regular basis.
So i don't think you should call the cops on that, but if they hurt the kid in any other way but the switch.. then maybe.