Do not start until you understand the service
Agree only to a service you can name. If you do not share a language with the shop, ask them to say what they will do before you lie down.
Name the service first
Ask what the visit includes, how long it is, and what it costs, in words you understand. A nod is not an agreement if you did not catch the service.
A shared language matters before you lie down
If the shop cannot explain the service in a language you both speak, do not start. You can look for a shop that can. This site does not translate the shop's menu for you.
You can stop
If the session is not what you agreed to, say so and stop. Read the services on the shop's website before you go.