I believe all custom fields can be translated, or are already translated by default. There is unfortunately one field that cannot be translated yet: the "Display" field, more specifically its HTML content.

(The title & description can be translated, but that is outside the purpose of this field: the title does not get displayed in forms and the description can't handle HTML with formatting or URLs.)

We use this field type to provide relevant information to users as they fill out our contact form (e.g. with a link to an external page), and we want its content to be translated just like the rest of the form and portal.