When visiting MS Knowledge base articles many are automatically translated (into Dutch).
I prefer the original English texts so that I don't have to second guess which Dutch terms the translator (human or machine) uses.
I'm using a Dutch Win7 OS with a Dutch Firefox browser.
No language or translation related add-ons are installed.
This SuperUser artice is only about IE.
This forum discussion suggests that it is a website (owner) decision, but is also IE oriented.
I do not want to disable all geolocation info as suggested here, because it's fine when e.g. Google search results are tailored to my location.
This article refers to an unreachable FAQ, searching for 'microsoft translation faq' gives me plenty of info about the MS translation services, except how to disable it!
[Edited to add sep 2014]
www.microsofttranslator.com to 127.0.0.1 (and to ::1) in the hosts file but that does not block the translation.