Google uses "geolocation" to determine where in the world you are, so it can give you a personalized experience. This includes using the language it thinks you prefer, and it also includes giving you different results on your queries.
If you really want to see Google in English you can click the link that provides that. But we consider this a feature with Tor, not a bug --- the Internet is not flat, and it in fact does look different depending on where you are. This feature reminds people of this fact.
Note that Google search URLs take name/value pairs as arguments and one of those names is "hl". If you set "hl" to "en" then Google will return search results in English regardless of what Google server you have been sent to. On a query this looks like:
Another method is to simply use your country code for accessing Google. This can be google.be, google.de, google.us and so on.