Everyone at Schiphol speaks English and nobody needs Dutch, which is true and also slightly beside the point. The thing that happens after a long crossing is not that your English disappears. It is that it becomes effortful. You can still order coffee and answer a border officer, but the part of you that handles nuance, humour and the small negotiations of a conversation has been switched off somewhere over the Atlantic, and what is left is competent and flat.
For a good many things that does not matter. For an evening in a room with someone you have never met, it can matter a great deal.
Where language actually shows up
Not in the middle. The middle of an hour of oil work is largely wordless, and a language you share imperfectly is no obstacle at all to it. Language lives at the two ends. The first ten minutes, where a stranger becomes a person and preferences get stated. And the last few, which set what the evening is remembered as.
That gives a useful rule of thumb. The shorter and more physical the booking, the less the shared language changes. The longer and more conversational the format, the more it decides everything. An hour is an hour in any language. Three hours, or an eight-hour overnight, leave long stretches for talking, and a first language in common stops being a nicety.
Just say it in the message
The roster runs to 45 therapists and the first languages on it are genuinely varied: Russian and the languages around it, French, Italian, Spanish and the Latin American variants, German, English of several kinds, Dutch. That range exists whether or not anyone asks about it, and most people do not ask, which is the only reason this note exists.
One line in the booking message is all it takes. I would rather this in French, if possible. It is treated exactly like any other preference, a length, a technique, a time, and it is answered honestly. Sometimes someone free at that hour speaks it and sometimes nobody does, and the reply says which along with everything else. The roster pages carry it too, so it can be settled before you write anything at all.
The booking itself is the easy part
Worth noting for anyone dreading a phone conversation in a second language at one in the morning: none of this is spoken. Booking happens by message, on WhatsApp or Telegram, and text is forgiving in a way speech is not. You can take your time over it, you can reread it, and the reply arrives in writing so you are not decoding an accent through a bad connection while standing in a corridor with a bag.
Send it in whichever language you find easiest to write. If it is not English it will still be read, and if you would like the evening itself conducted in it, say so in the same breath.
What this is really about
A body on the wrong clock is already spending most of its capacity on being somewhere unfamiliar. Every additional small effort, the currency, the layout of the building, the switch of an electrical socket, the language, takes something from what is left. Removing one of them is not a small thing at the end of a fourteen-hour day that has crossed five time zones.
The currency question is handled the same way, incidentally: card, crypto, or cash in euro or another major currency at the exact sum confirmed in writing beforehand, for the balance settled on arrival. Two frictions removed rather than one. The rest of the practical detail sits on the business traveller guide, which was written for people arriving at the end of a working day rather than the start of a holiday.