He has to express his writings in well-articulated words without being offensive in content, but very straight-forward and clear in substance.
This means that, any letter he is sending either to his home, state or receiving state must be carefully scrutinized to a void improper usage of words, consciously or unconsciously, in what he intends to convey. In summation, a diplomat must judge accurately and appropriately the likely behavior, reaction and actions of others, and appreciate their views with a clear sense of accuracy and equally present them to his government.
This is the best way to ensure mutual understanding between states for continued cordial diplomatic relation.