If you want to modify the text viewed as "Show as slideshow", you've got to go to Gallery --> "Options" --> Select the tag "Gallery" and then you'll see into the fourth option, two customizable fields: "Show as Slideshow" in the first field, and the second field "Show picture list".
You've got to put "Mostrar como diapositivas" & "Mostrar lista de imágenes", then "Save Changes" and you will see your translation :)!
And another little thing. When you are talking about objects, animals, etc.. you've got to use "does and doesn't".
I, You, We, They --> Do
It, She, He -> Does
"..But the translation don't work" is wrong because translation is an object,
"..But the translation doesn't work" is how it should be.
Sin nada más.. saludos desde Argentina!!