When using PayPal payments standard, there is no way to override the language shown by PayPal when you are pre-populating the PayPal checkout pages with the payer information where you specify the billing country, as this takes presedence.
If you were not specifying the billing country in your PayPal form submission, you may set the language shown to the shopper by including the form input parameter lc having with a value GB for your British customers living in Germany. This will then given them an English language checkout page in PayPal. They would then set their billing address themselves on the PayPal page during this part of the checkout.
<input type="hidden" name="lc" value="GB">
So if you can live without pre-populating their billing address when you transfer them over to PayPal, then you will be able to control the language.