外国のホテルでチェックインする際に、用意が必要なものって何？って英語でなんて言うの？

"What will I need to prepare?" or "What will I need to provide"is a way of asking what you need to do, or what you will need to be ready to give to the staff at check-in.

"What are the requirements?" is a way of asking what is required or what items are required of those checking in.

Check-in is the term for the time or the action of checking into a hotel.

In order to make sure you have all the requirements when checking into a foregin hotel, it would best to say "What documentation is needed to check into a hotel?". This outlines that you wish to know what is needed in order for you to have a successful check in.

*What documentation is needed for checking in a hotel.- the word documentation includes any documents that will needed for checking in. This will include your ID,invoice for the booking etc.
For example:
A:What documentation is needed for checking in a hotel.
B: They will need only your ID because they will be able to pick the rest of your information in their database.

When going to a hotel to stay you usually have to provide/show documents such as your passport for security and to check you into the hotel
To provide means to give something or to show them something
Documents would be a visa/ID/passport etc
To check in means to sign paperwork to get your room

When talking about staying at hotels abroad, and you want to ask someone what you need to prepare when you want to check in at a hotel (such as an ID etc), then you may apply one of the above exampled sentences.

easy and casual ways of asking what the hotel will need in order to 'check in'.
'Check' is used to describe to see or observe something ex. passport/ID and reservations.
'Requirements' are used when describing thing that will be used/needed to complete something, in this case to check in to the hotel.