Add German language to an existing Webshop

We want to add German language to our existing webshop. The webshop is based on oscommerce and is heavily modified with some standard add-ons and some customized add-ons. A main point is that the prices for the German customers has it owns VAT. Currently our webshop uses Swedish VAT (for EU) or no VAT (for countries outsides EU). For Germany the prices should be with 19% VAT. In addition to that we need to create a first page where the customer chooses the language/location applicable for him/her in order to reach the correct pages (language and prices).