If you are referring to the tooltip text which appears when hovering over the navigation buttons on the Button Bar (displayed by setting showNavButtons="TRUE"), then you can change the text using the languageList configuration option.
For more information about languageList , please see here.
This configuration option is not found in the JuiceboxBuilder-Pro interface but can be added manually following the 'Setting Config Options' instructions here.

The default languageList which you can change to suit your own needs can be found in the link in my last post: http://www.juicebox.net/support/creation/#language-list

If you want to change the tooltip text for the Button Bar navigation buttons, use something like the following (changing the text as required):

languageList="Show Thumbnails|Hide Thumbnails|Expand Gallery|Close Gallery|Open Image in New Window|Images|TEXT_FOR_NEXT_NAVIGATION_BUTTON_GOES_HERE|TEXT_FOR_PREVIOUS_NAVIGATION_BUTTON_GOES_HERE|Play Audio|Pause Audio|Show Information|Hide Information|Start AutoPlay|Stop AutoPlay|AutoPlay ON|AutoPlay OFF|Go Back|Buy this Image|Share on Facebook|Share on Twitter|Share on Google+|Share on Pinterest|Share on Tumblr|of"