Labels are shown as they appear in the Spanish version of the console.
In the person's profile. Open the person in Personas, Contacto (Contact) tab, Más información (More information) card, Información Personalizada section. The section only appears if the person has a custom field.
In the people download. When you download with Base de datos en tiempo real (Real-time database), each field comes out in its own column.
In your emails. In an autoresponder you can show the value with the merge tag {{customFields.<field>}}, replacing <field> with your field's name.
Note: in landings, custom fields don't work as variables: the donor would see the text as is.