var translatableElement = '<div>' + Drupal.t('Translatable Text') + '</div>';
The easiest way to do this in Orbit is to move the variable "directionalNavHTML" into the "defaults" object, and changing all calls for "this.directionalNavHTML" to "this.options.directionalNavHTML". This has the slight disadvantage that anyone implementing Orbit who changes the setting may break the nav (e.g. by assigning classes other than 'left' or 'right').
On the other hand, it would be possible to provide overridable defaults just for the actual text 'Left' and 'Right', but since the directional nav markup is provided before the options object is merged with the defaults object, it would necessitate rearranging a bunch of code.
Note: according to Github, I should be able to attach the patch, but I can't. Presumably I don't have permissions to do so. You can download the patch from Dropbox.
#252 Allow orbit nav button text to be customized
Fixed in master.