Unfortunately, neither JuiceboxBuilder-Pro nor the Lightroom plugin feature support for the languageList configuration option in their interfaces.
(It is possible to include a custom languageList in WP-Juicebox as the gallery settings window has a Pro Options text area where you can manually enter any configuration option.)

However, after creating a gallery with the Lightroom plugin, you can open the gallery's 'config.xml' file in a plain text editor and add the languageList option as an attribute to the opening <juiceboxgallery> tag.

Alternatively, if you want your custom languageList to appear in all galleries created by the Lightroom plugin, you can modify the template file that Lightroom uses to generate the 'config.xml' files.
Open the 'juicebox_pro.lrwebengine/config.xml' file in a plain text editor and add your custom languageList on line 34 (after the opening "<juiceboxgallery") which is currently blank. The line number refers to the current version of the Lightroom plugin (v1.4.4.2).

For reference, more information about the languageList option can be found here.
Instructions for how to manually set configuration options can be found here.