Sort in multi-language text field

[Orkun]

I have a find statement where I am doing 

sort=doctors_last_name

The field is a multi-language text field. It is not required.

The doctors should be ordered alphabetically.

The problem is now, that the field is sometimes empty in the default language and because
of that the doctors with the empty lastname get prepended to the beginning.

Sorting in memory is not an option, since I am using pagination and there are also over 2000 Doctors.

How does the sort in selector exactly work? Does it look first in the current language and then as a fallback to the default language?

KR
Orkun