"You have joined a noble cause, my friend."
-- Grindelwald to his servant Abernathy
Grindelwald rewarded his loyal servant Abernathy with a new forked-tongue for helping him escape the Aurors who were transporting him from MACUSA in New York to Europe. Abernathy and Grindelwald had switched appearances at MACUSA so that when authorities decided to remove the tongue of the chained but dangerous Grindelwald, it was actually Abernathy who became tongue-less.
Replacing Abernathy's normal tongue with a forked-tongue speaks to the evil nature of Grindelwald, and also foreshadows the next villain in the Wizarding World, Lord Voldemort, who was obsessed with his pureblood Parselmouth heritage symbolized by the Dark Mark, a human skull with a snake emerging as a tongue.