Since he is a diplomat I expect that the ability to translate is sometimes more important that his own understanding. He doesn't directly have power to make decisions, he needs to bring everything back to the leaders of his country. (of course he will know what the goals of his country are and what compromises are acceptable, but his decisions are subject to review)
We do a lot of on the fly translations. In meetings, we may have to translate for an Ambassador or senior official that doesn't speak the language. Depending on the topic, it may not be appropriate to bring a professional translator, so we do the best we can.
We also meet with a wide variety of people and take copious notes. I personally take notes in a mix of English and the local language, but mostly in English because I have my own shorthand to help me remember what is said. It's hard to develop that kind of shorthand in a foreign language without writing everything and that is too slow for a normal conversation.
Actually, I often find that I can fluently exchange with someone speaking English (which is not my native language), but if a third person ask me to translate, it's really a higher level of complexity to find the relevant expressions.
I understand that at the start, one feel more at ease to use its know point of reference in languages already known, but at some point this scaffold is only an additional layer that slows down your expression flow, isn't it?