Perfect,
As nowadays I only have "No language" (Ghosting Test.pdf), English and Spanish books, I've used the next search:
Original books:
Code:
(#language:False AND #original_language:False) OR (#language:"English" AND #original_language:"English") OR (#language:"Spanish" AND #original_language:"Spanish")
Translated books:
Code:
NOT((#language:False AND #original_language:False) OR (#language:"English" AND #original_language:"English") OR (#language:"Spanish" AND #original_language:"Spanish"))
And both work perfectly. I've just saved them as searches/restrictions.
When I learn German, Russian and Chinese, I will think about modifying my "not-so-perfect" search 