You can activate / deactivate registering customers from Configuration -> Settings -> Clients -> Allow clients to register.

Your customers will be able to subscribe to your base URL your-company.djaboo.app

If customer registration is disabled, you will only be able to add customers from the admin area.

The language drop-down list is an indicator of the language the contact will use when browsing the customer area.

Keep in mind that the contact language only applies to the customers area, this means that all emails, PDF documents will always be sent from the default language selected in the customer/company profile.