Hello and welcome to Tom's Hardware Forums.
That's either one big installation error or a system you bought without looking at it first. Whichever, it's going to need reinstalling if it was a native German system, when it was first set up. You can add English UK or US as a language in Control Panel>Regional and Language but if you can't read German, that's not an easy place to find.
Can the person you bought it from not help you here? You did buy it - didn't you?