Picture the person your country sends to sit at another country's table. They can't give orders and they can't force anything — their whole job is to talk, to read the room, and to keep two sides that may not like each other still speaking. That skill of handling a delicate situation with words alone is what carries the second meaning too.
Two branches
職務：外交官
核心畫面本身：那個被派駐在外、代表國家說話的人。
diplomat (official) · noun · B2
someone who is officially sent by their government to represent their country in another country, for example to talk with that country's government or to help their own people who live there.
外交官
性格：會做人的人
把外交官的本事從國際場合搬到日常人際：一樣是靠說話讓緊繃的局面不炸開。
tactful person · noun · B2
someone who is good at talking to people and handling difficult situations, so that nobody gets angry or feels hurt.
圓融的人；和事佬
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
diplomatnoun/ˈdɪp.lə.mæt/
A person, such as an ambassador, who is accredited to represent a government officially in its relations with other governments or international organisations
Someone who uses skill and tact in dealing with other people.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。