You may need to delete your cookies (at least all those pertaining to Google) before you can get it to stick.
I haven't had your problem with google.com (the US version, which is where you want to go), perhaps because I keep that link, as well as the links google.uk and google.fr on the links bar on my toolbar.
But I did have the same problem once with the MS support website: every time I tried to search there, it took me to a page in French. Had to delete all my MS cookies before I could get it to default to the US site.