I am an interpreter, I seldom (or never) do translations. Project history does not apply to me. How can I complete my profile?

If you go to the "Add project" page, you'll see that one of the options in the drop-down box is for Interpretations (even though translation is the default), so you could enter a project based on an interpretation job you have done as well, since this will also give potential clients a better idea of the type of work you do.