Very similar to how everyone in ancient Rome spoke with an English accent.
I always loved how Richard Burton impersonated a German officer to get into the Schloss Adler in Where Eagles Dare and spoke with a British accent while all the other Germans spoke English with a German accent. He didn't even fake it. And none of the other Germans noticed at all.
"Do you know who I am? I'm Richard Buuuurton dammit. I will not fake a German accent. Not for any director. Because I'm Richard Buuuurton!"