If I open a new tab in Chrome (my current browser of choice), then type google.com into the address bar and press return, I am redirected to google.de in German.
If I type in a random query in English, e.g. "football club", I see a mix of German and English results (including some news results from a paper close to where I come from in England - scary!)
I also have the option of selecting "Google.com" at the bottom of the page. If I click this link, I am redirected to google.com and all of the results for the same query are in English.
I tried the same thing in Firefox and I am also redirected to google.de.
Shame. that would have been a reason to go back to using Firefox.
In fact, I just checked. I get redirected in every browser - Opera, IE, FF, Safari and Chrome.
FWIW, Google isn't the worst offender as even google.de offers results based on the language of your query (your original point, I believe).
There are other sites which "helpfully" direct you to the pure German version of their site, leaving you to hunt for an option to switch to English.