jul3 on "[Plugin: NextGEN Gallery] "Show slideshow" bilingual"

i've searched the support forum already, what i found was how to change the default "show slideshow" and "show thumbnails" to another language. but i want it to work with two languages. i'm using polylang, and i can translate gallery titles etc with it, but not these strings. how and where can i accomplish this? do i need to edit the po file?
any help to point me in the right direction is appreciated!

https://wordpress.org/plugins/nextgen-gallery/