Does saying "Verify you're a human" in the heading of a captcha make sense? As far as scripts, bots, or other automated spam are concerned, they cannot make anything of this. At most, they would just know that there's a captcha and feel (the irony) like there's a layer of protection.
As a captcha is essentially for separating two kinds of users: humans and automated spam agents, there cannot be a third entity.
I've user tested a captcha precisely one time, but I'd argue that yes, it does make sense. Many, many people still don't know that bots are capable of submitting data via forms. Therefore, they are also unfamiliar with the concept that bots submitting forms is undesirable. And they are thirdly unfamiliar that tests for sentience exist and are necessary.
Because of all that, you need an instruction with a captcha.
Many first-time web users are 60+, and I've seen them struggle with understanding the difference between AOL, the internet, and their email addresses.
Show my elderly father a captcha, for example, and he knows he has to do it but he doesn't know why. He's not new to the web. He knows just enough to trade stocks, to email people, and to read Fox News. He usues captcha but doesn't know why it's there; he certainly doesn't experience a sense of security when he sees it. He experiences annoyance. We've talked about captchas before; he knows he has to prove he's a human, and therefore that bots are capable of filling out forms, but he doesn't know what can result from that or why it's undesirable.
TL;DR: Not every user is a 20-something who grew up looking at captchas.
My guess would be that it's a more "positive" prompt than something more antagonistic like "verify you're not a spammer". The latter is accusatory, but the former is- as you point out- obviously true.
You say this so that users know why they have to do the test. It's irrelevant if bots can read it or not.
When selecting a language on a web page - I don't understand what language 官話 is (and I don't have to), but a Chinese/Mandarin speaking user would find it useful.
Actually a nice point to talk about. With the trends in development using a few image processing techniques, a bot can also be in a position to crack the captcha but will have troubles if the space between the letters is very less or when overlapped. This is how you distinguish between humans and bots(basically programs) with captcha which is required at some points.
You use that phrase because you are talking to a human and nothing else. If something else needed to read or understand that phrase, it would be done differently. That is the answer to the question.
The phrase is as user friendly as any explanation/descrpition on any other input field – such as "Please enter a valid email address" or the like. So one should look at it as what it is: an explanation/descrpition, of what the user is expected to do with that input field. In my opinion such explanations have more to do with 'corporate language' or 'branding' and how you want to address your audience than with the question, whether the audience is expected to be familiar to the concept, or not.
Of course, you could probably shorten all those messages to single terms like "email" or "captcha" – but depending on your target audience that could probably concidered not a very user friendly practice.
The fact that robots won't 'care' about that explanation has nothing to do with the original purpose of that message: explaining the (human) user what to do.
