Yes, a bathroom must be ventilated to the outside, if not possible directly then via forced ventilation.
Have a read of this recent thread from another forum http://www.andalucia.com/forums/viewtopic.php?t=7802
The document you need to refer to is the Codigo Tecnico de la Edificacion, Part DB-HS: Salubridad, Seccion HS 3 Calididad del Aire Interior http://161.111.13.202/apache2-default/cte/CTE_DB-HS.pdf
It’s a huge 144p pdf doc, the relevant part is at page 65 onwards.
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