not typical of or suitable for diplomats (= officials whose job is to represent their country in another country, and who usually work in an embassy)

This was an unprecedented use of undiplomatic language by the veteran diplomat. 这是这个经验老道的外交官史无前例地使用非外交辞令。

acting in a way that might cause people to be upset, hurt, or annoyed

Nobody can accuse this book of being undiplomatic; the author is unfailingly kind. 没人会说这本书不通人情;这（书的）作者一贯是一个温和体贴的人。