Short answers, pulled from the story.
Lancelot de Carle wrote the poem in 1536. He was the secretary to Antoine de Castelnau, the French ambassador to England, and was an eyewitness to Anne Boleyn's trial and execution in London.
The poem details the life and the events surrounding the arrest, trial, and execution of Anne Boleyn and the men accused with her in 1536. It runs to 1,318 lines and was written in French.
Lancelot de Carle dated the poem the 2nd of June 1536, shortly after Anne Boleyn's execution. It was not published until 1545, though it circulated widely in manuscript before that date.
The men executed with Anne Boleyn were Mark Smeaton, a royal household musician; courtiers Henry Norris, Sir Francis Weston, and William Brereton; and George Boleyn, Viscount Rochford, who was Anne's brother. All were convicted of treason and adultery with the queen.
The poem was written in French. Its full title translates into English as A Letter Containing the Criminal Charges Laid Against Queen Anne Boleyn of England.
Yes. De Carle was in London in May 1536 as secretary to the French ambassador, and the poem's account is that of an eyewitness to both Anne's trial and her execution.