"An ambassador is an honest man sent abroad to lie for his country." - Henry Wotton, Sr.
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"Successful career diplomats... seem to be able to both send and receive with a level of precision and detail not found in normal discussion...
Diplomats will use the precise word in their interventions that conveys the meaning that was intended. They will differentiate between words such as would, will, could, or should. They will say persuade or dissuade as appropriate. When they write a reporting cable, they will report if the intervention they listened to indicated the speaker indicated his or her nation would oppose, object, or be disappointed, as each of these would be taken differently (oppose means they would work to block it; object means they would not like it but would reserve any commitment in terms of action; and be disappointed means they do not like it, but they would accept it without blocking it).
In some cases, subtle shifts of policy or concessions within negotiations are signaled by the change of language from one meeting to the next. In the case of the UN General Assembly, with its annual debate of topics, this means that the listener must know that last year a nation indicated it would oppose a proposal whereas this year it would only object.
Diplomats... understand the effective use of the subjunctive... In some cases, where they each want to come to an agreement over language that each of their capitals will find acceptable, they use what is called constructive ambiguity so each may claim the meaning of the language was what they intended.
Diplomats do not like to simply say “no” on something. A diplomat might respond to a proposal by saying “maybe,” but it is understood that this means “no.” If the topic is more complicated, they may say, “This is something that requires careful study and consideration.” If a proposal is so annoying and complicated as to not even merit study, the diplomat might say. “We will need more time to give this the level of attention that it deserves,” which of course means “none.”
Diplomatic language is understated, and the delivery of spoken interventions is almost always done in a polite unemotional voice. When a diplomat refers to “all means necessary” or “face serious consequences,” these are understated ways of referring to the use of force—war. During multilateral meetings diplomats will typically not address each other directly, especially if there is disagreement. An intervention will typically start out with the words “Mr. Chairman” or “Madam Chairwoman,” and the grammar of all sentences spoken will be framed so the speaker is addressing the chairman, not the other diplomats. If the French delegate wishes to challenge the U.S. delegate, he or she will not say “I disagree with the U.S. position that . . .“ but will instead say (in French) “I would take exception to the position voiced by one nation that. .. .“ Where there is agreement the diplomats will be more direct and say, “I wish to echo the views expressed by my distinguished colleague from Russia that. . . .“ When a diplomat wishes to make it completely clear that he or she is about to express the key points of an official position he or she will preface these points with “on behalf of my delegation,” but when he or she wants to signal that the views about to be expressed are official but not necessarily to his or her liking, the diplomat may interject “on instructions from my capital...”... If a diplomat is speaking at the United Nations and his or her native language is one if the six official languages at the UN (Arabic, Chinese, English, French, Russian, and Spanish), he or she will essentially always speak in that language, even if he or she is fluent in one or more other languages...
Although all of this subtlety of language and nuanced meaning may seem at first an impediment to clear communications, the opposite is true. Diplomats develop a keen sense of the psychology of communications. They understand each other... Rarely, if ever, are wars fought over misunderstandings between diplomats."
--- The Subtle Language of Diplomatic Discourse in The psychology of diplomacy / Harvey J. Langholtz, Chris E. Stout