Thank you for these links to Isaac Don Levine's work, Dominic! I noticed that these web pages include a few links to pictures of excerpts of the original letters, showing that Wilhelm wrote in the English language and that the letters presented are not translations.
It also appears that these letters do not include the telegrams in
The Willy-Nicky Correspondence as edited by Herman Bernstein, but rather supplement
The Willy-Nicky Correspondence.