Many words and sentences are predefined in the system, often in multiple languages already. These system texts work through what we call 'strings'.
A string can be the text "Number of participants" in the booking form, above the drop down. And also the choices "1 participant", "2 participants", etc. are strings that the system will display by itself.
To create a multilingual system, most of these strings are translated. It's possible however to adjust the translations of all these strings and even the 'original' English strings.
This can be helpful if you want to change specific words. There might be a better choice of words available in your language or your school's specific situation.
In Settings > Translations, you can select a column language on the left side and right side.
Use CMD + F (or Control + F on Windows) to find the string you want to adjust and change it in the column on the right side.
It's possible to set both columns to English if you want to change the strings in English.
Alternatively, set the left column to English and the right one to Nederland if you want to change the Dutch translations.