In the books, J.K. tends to write almost all foreign characters with their dialogue including the pheonetics of their accent, yet we hear Grindelwald speak to Voldemort, he speaks no differently than any other British characters.
Do you think this is because they were 'speaking' German, or does Grindelwald just not have a very heavy German accent? Why do you think this is?
I haven't read DH recently enough to comment on whether Voldemort speaks German, though given what Tim has written, I would doubt that Voldemort does. Though he is enough of a git to kill someone for talking to him and being a Muggle, especially foreign.
Grindelwald could just speak very good English. He was related to Bathilda Bagshot, it's entirely possible that he spent other holidays in Britain, or that one of his parents spoke English and taught it to him as a younger child.
Being casually cosmopolitan and speaking several languages and dialects fits how I imagine Grindelwald anyway. It fits how I imagine his rise to power as a charismatic, charming, seemingly reformer type. I see him being the type of person who would work a diplomatic gathering by speaking to everyone in their preferred language and switching flawlessly between them.