Custom fields for events cannot be translated on the Event info page since no translate icon is shown

To reproduce on the 4.5 demo server:
Enable multiple languages and add a second language to the database.
Create a custom field set that applies to Events of any type.
Add a field to this this custom field set such as a text field.
Edit the Rainforest or any other event.
The main event fields like "Event Title" have the translation icon (Two little green/grey squares) but the custom field does not.