Can I just clarify something, you want to put your machine into German mode to test your web application, which just happens to be running on the same machine as the browser?
When this is running for real can we assume that the sever could be hosted outside Germany (or maybe in Germany but on a machine set up with a US locale - it happens a lot). If so setting the culture on the server isn't really what you want, and you probably won't be able to do that when you deploy anyway.
You also have another problem, there is no fool-proof way you can detect which culture your user wants to use. You can detect all sorts of info from the HTTP headers, do a domain lookup on the IP address, but what if a German speaker wanted to use your site from the US, or an American was in Germany and wanted to see "." and not "," for the decimal separator?
To put it another way, I am saying it doesn't matter what culture your client's PC is using - it is not relevant because once you go over the web all bets are off - even if your app did detect your culture correctly it might not be what your user wants!
So, the way I would do this is to ask the user what language they want and then explicitly set the locale in your application at run time. This does mean asking the user to pick a langague but it's the only way to be sure of using the right locale for the user. If all your users are German then you don't have to ask, you can just hard code it. If his approach sounds like it might work then there's some good sample code here: http://msdn.microsoft.com/en-us/library/bz9tc508.aspx.
Good luck, or rather Viel Glück!