There is a translation bug in gnome-packagekit that affects versions from 0.2.5 (at least) up to 0.3.10.

In the Spanish translation file

#: ../data/gpk-signature.glade.h:1
msgid "<big><b>Do you trust the source of the packages?</b></big>"
msgstr "<big><b>¿Confía en el reospitorio de paquetes?</b></big>"

Should be
#: ../data/gpk-signature.glade.h:1
msgid "<big><b>Do you trust the source of the packages?</b></big>"
msgstr "<big><b>¿Confía en el repositorio de paquetes?</b></big>"

Note reospitorio (wrong) -> repositorio (correct)