Most probably the BIOS will be in English, because that's the universal language in IT. It can be changed, if necessary, by a BIOS update, via motherboard manufacturer.
Windows 7 may come in German, but probably you can order it installed in English, it wouldn't be a big deal for the seller. Windows language changes are only guaranteed in Ultimate and Enterprise, so getting yourself a Home, will lock you with Deutsch.
U can´t change the language in BIOS. Most BIOS updates comes in Eng, and doesn´t support other languages.
And u CAN change the OS language, just download a language pack from Microsofts website, or with Windows update.
Search for updates with Win Update from the Control Panel, then check the menu "Optional Updates", and choose German Language Pack and click Install Updates.