カフェの隣にあるトイレを使ってくださいって英語でなんて言うの？

"There is a restroom next to the cafe, feel free to use it."
"Feel free" is a expression used to describe permission given to someone.
"Feel free to make yourself a cup of tea".

When restaurants or cafes are situated in a shopping mall, then the restrooms aren't inside, it is normally a public restroom that all shoppers can use. To explain to patrons/clients that you do not have a restroom inside the cafe, you can use the following:

"The restroom is next to the cafe!"
"We don't have a restroom, it is next to the cafe!"

If there is no restroom in your restaurant for customers to use you can say "We don't have a restroom, there is one next door in the cafe however.". That is the most polite way to let someone know. You could also say "There is a public restroom in the cafe next door that you can use.". It is more straightforward but it is acceptable.

You can simply explain to the customers where exactly the restroom is and add 'please use' or also 'feel free to use'.

'Feel free to use the restroom next to the cafe'.

When describing where something is it's always good to give precise instructions so the person knows where to go
so if something is next to something then you could say 'next door' expressing that it is not far away

In this phrase you start by telling the person why you wont allow them to use the toilet. Then the next part of the sentence tells them where they can go.

You can also add the phrase 'feel free to use the toilet next door.'

Often in English the word toilet is interchangeable with the word restroom.

When asked where the restroom is, you want to explain that you don't have a restroom in your restaurant/store, but there's a public restroom next to the cafe next door. You want to say something like "Please use the restroom next to the cafe"
WC = toilet/restroom