Bookly is compatible with the WPML plugin, which gives you translations into dozens of languages and lets you tweak the wording of all of it.
It translates dynamic values as well (categories, services, etc.).
The front-end booking form can be translated via Appearance section in Bookly menu.
You can help us improve Bookly by contributing translations for the languages you speak. For more details, please reach out to us via firstname.lastname@example.org.