You can find the swagger documentation on https://payment.apaleo.com/swagger/index.html.
All apaleo APIs are described as Swagger documents. That lets you generate API clients directly from the swagger.json files. There is a large community providing client generators for almost every language. We strongly recommend using AutoRest, to get proper handling of Enums in the API. AutoRest allows the generation of API clients for Java, PHP, C#, NodeJS and more. This quickly gets you up to speed, and as a bonus, you can access API documentation within the auto-generated methods and models.