After spending a long time looking, the best out of the seriously sparse set of options appeared to be Visual Captcha (https://visualcaptcha.net). Unfortunately it seems it's no longer under active development.. Fingers crossed someone picks it up or something new comes along. A good, open, customisable, and privacy-respecting captcha open that developers deploy in place of ReCAPTCHA is desperately needed.
Seems to be PHP, JS and Ruby only.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!