Ok, my issue was that the reCAPTCHA wasn't showing on forms when in other languages than default.
For a short story, I'm just beginning my setup of Drupal, the is by default in french but also provide english and spanish.
The first issue I remarked was that on the default french viewing, the reCAPTCHA was showing in english, I first thought it was due to a missing configuration option on my reCAPTCHA google setup...
But I got really worried when tuning thing for english and spanish as the reCAPTCHA box picture wasn't showing on these pages although the CAPTCHA form include was there with the right text language of the configurable challenge sentence.
I found my solution by unchecking the "Use the AJAX API to display reCAPTCHA." that was set "On" by default at install in the reCAPTCHA setting tab.
This resolved both problems, the reCAPTCHA is now showing whether we are in the default or other languages, but also show it's box picture in the right language asked to Drupal.
So, I think the issue would be to look at this AJAX use to see why it doesn't handle well other languages...
Thank you for all those who worked on this usefull module!
(I'm volunteer to test an eventual patch...)