Relationship between two microservices with different database by means of api rest
You maintain relation via you services. All you have with you is two services, Client Service and sale service. 
Client service ideally should have no information about sales service, it should not know even if the sale service or any other service even exist.
However for you to record sale you need to do it against client, and in that case sales service can keep client id and can pull information from client when needed.
It is upto sale service to decide what relation ship is, it can allow for one client to have multiple sales record or one. Service can decide that, This might not exactly be required to be embedded into database by means of FK.
Sales service can keep all the information related to sales alongside an id (Just like you would have in traditional database). Based on database choices you can put constraint against this id if you have to.
How can I make a relationship between two APIs, where an application is in node.js with the mongodb and another in python with mysql, I need to make a relation between them of 1: N between client:sales, in one I have the register client (name, cpf) and the other part is a sale, but for me to carry out the sales I need customer data.
In a traditional application this relationship would be through a foreign key, as we are talking about two separate applications how could this relationship be made ??