You can customize BitNinja Captcha pages in the www directory.
The files you can have:
 captcha_challenge_template.html - The challange file to server

You can have internationalized versions of the challange file.
Example internationalized challange file names
 captcha_challenge_template_en.html - English version
 captcha_challenge_template_gr.html - Greek version

You can also have static files and they will be server untached.

Don't forget to restart bitninja after editing the customized files
because BitNinja caches all content into memory to speed up responses.