Hello @kostasb30, @supergrower
You need to use buddypress-media.po file with poedit for the translation.
@supergrower, We have updated our text-domain from rtmedia to buddypress-media because WordPress stated that plugin’s textdomain must match its slug. Reference link
In order to make it work, the only required thing is to update text-domain in your theme as well. Please make sure your theme uses same text-domain in template pages.
Thank you, Pranali