That's because there isn't a path extensions/recaptchaextensions/recaptcha/lang/English/.
Somehow, when I un-installed/re-installed the extension, it left part of the path somewhere and appended to it: extensions/recaptchaextensions/recaptcha/lang/English/.
Anyhow, I've ironed it out so that it doesn't throw errors anymore, but it still doesn't work. Nothing changes when it is installed - no options appear to configure it where they should, no captcha shows up on the register screen...
I'm sure it is just something I've screwed up when I modified the files. I don't know enough about PunBB and it's extensions (or PHP) atm to fix this.
Here are the files including my modifications in case anyone wants to take a stab at it (mannungo?).