On Youtube I have seen a lot of videos like this with car accidents in Russia recorded by in-car cameras (dashcams). Is it legal to use such a device in Germany? If an accident happens, can I use the video as a proof?
This device is legal as long as you do not publish the video. In case you want to, you need to pixelate license plates and faces. Also you are not allowed to have a specific person "in focus", meaning: You are not allowed to sit in the car and point it at specific people. Regarding your comment, having a camera in there that does not record is perfectly fine, no need to remove it.
You can present the camera in Germany in case there is an accident but the validity of the video as a proof has to be decided by the judge. So while there is no guarantee, the chances are high of course that this will help you a lot.
Be careful though: If the police takes the camera as a proof and finds you crossing red lights earlier in the video, they can take that as a proof to get you in return.
I think it is legal to have a in-car camera installed while driving, not only Russia but also the US and the EU, there are several companies that will give discounts on insurance by having "black Boxes" fitted to the vehicle, These are mostly aimed at new young drivers as their insurance quotes tend to be shall we say "HIGH".
I have fitted 2 HD car cameras at the front and rear in my car, these have recorded some very close calls (not with my vehicle) but with others in front of me. I have fitted camera's in my cars for a few years now, upgrading the camera's as new ones come in.