Appointments are very easy to translate and the translation process is pretty much the same as for any other WooCommerce Extension. The simplest way is to use the Loco plugin with WooCommerce. The other way is to translate text strings WooCommerce translate manually with Poedit.
Where are language files located?
The language files for Appointments can be found within /wp-content/plugins/woocommerce-appointments/languages/. The default English language file is named woocommerce-appointments.pot. Use that file for further translation.
How to translate?
Using a program like Poedit, you can use the .pot file to create language translation files. For example, if you want a translation of the Appointments extension in Dutch from the Netherlands, you would translate the strings and create translation files: woocommerce-appointments-nl_NL.mo and woocommerce-appointments-nl_NL.po. Those two language files would then need to be moved into the directory: /wp-content/languages/woocommerce-appointments/.
The other way is to use Loco plugin or any other plugin, like WPML – if you intend to use WPML, make sure to also install WooCommerce Multilingual plugin, which is fully integrated with Appointments and allows you to have multiple languages installed on your site at the same time.