The following system messages refer to a CAPTCHA to be calculated (SimpleCaptcha and MathCaptcha). This does not really fit for other CAPTCHA modules like FancyCaptcha, and QuestyCapcha etc.
- MediaWiki:Captcha-createaccount
- MediaWiki:Captcha-badlogin
- MediaWiki:Captcha-sendemail
- MediaWiki:Captcha-addurl
I guess it will be great to have these messages less CAPTCHA module specific.