HI Norberto,
Thank you for your inquire.

• Does Amelia have a module where I can see the history of each client

Unfortunately no, the only thing that is visible in the Customers page is are some basic information like you can see it on this link.

• How do I mark an appointment as completed?

At the moment our plugin does 4 statuses for the appointments. 

Approved, Pending, Cancel and Rejected. We do not have status for completed appointment.

• Is it possible to have a version of Amelia in Spanish?

Yes we do have Spanish translation, you just need to choose Spanish as a main language in the WordPress Settings.

You can check out how our plugin works and looks like on our demo sites

https://wpamelia.com/demos/