Since 11.5 version of Presence Builder Captcha Translation may be changed through the main menu of Web Presence Builder (Settings > Languages: Website language).
This property affects localization of certain website elements related to third-party components such as
If you use Web Presence builder version 10.x or 11.0.x, please consider upgrading your WPB edition.
Resolution for 10.x and 11.x
Since the issue requires complex manipulations with the database, it is recommended to contact Plesk Technical Support for assistance.
Additional Information for Plesk
It's by design that "Web Presence Builder" uses the same set of locales as Plesk.
Refer to The Guide to Localizing Plesk 11 for more detail about panel locale localization.
In Presence Builder 11.5 Localization Guide you may find the detail about WPB locales customization.