チェックイン前に荷物を預かってくださいって英語でなんて言うの？

A lot of hotels have a room where you can leave your bags. You can use either of these examples to ask if you can just leave your bags in the back room or a room they have for guest bags.

"Could you....." is a great way to start a request. To be slightly more polite you may start the request: "Would you.....?" You may add 'please' to the request also to soften the request: "Would you please...?"

Usually hotels have some sort of holding area for bags. So, you can just ask if they can hold your bags until you check in.

Letting the hotel clerk know your "reservation time" then asking "can you check my bags until then" gives them enough information to know your reservation is  until later, but you need the service now. Asking "can you check my bags until my reservation" is a polite way of soliciting the service ahead of your check in time. Saying "please check my bags until my reservation time" is a forward but polite command. It is okay to say this especially if you know the service is performed at the hotel.

These questions allow you to ask if the hotel is able to hold your bags until check in. You would use "can you" if you already know that they have a place to hold your bags. If you are unsure whether they hold bags prior to check in then you would ask if they hold guests' bags until check in.

"Could you hold my bags for a while?" This is fine but it does not give any extra details about your situation. Sometimes adding details to your sentences helps other people understand you better. "Could you hold my bags for me while I wait for my check-in time?" This sentence asks the hotel to hold your bags but also lets them know why you are asking. Because the hotel can understand more details about why your asking they may recommend good places for you to rest before your check-in time.