Originally Posted by Aviatrix

Tripadvisor isn't a booking site. It just acts as a search engine - it searches booking sites and then brings them up in separate windows. If it thinks you're in Germany then it sends you to German booking sites - you don't get any choice in the matter, they just pop up. Some (like booking.com) will let you switch language and currency once you've got there, but you can't - for example - switch from expedia.de to expedia.co.uk because you will lose the results of Tripadvisor's search.