Are there languages/software that implements http status code 418?

Yes, it is implemented (by a teapot).

This error code is an impotent part of HTCPCP(Hyper Text Coffee Pot Control Protocol).

Websites that have implemented it

Google - www.google.com/teapot - (Details from @ButtleButkus)
Stackoverflow on CSRF violations - Meta question about it - (Details from @ImmortalBlue)
A teapot - joereddington.com/projects/418.. - (Details from @MegaTom)
Snarked - www.snarked.org/coffee - (Details from @MR.X)

res.send(418)

Sends following HTTP header:

HTTP/1.1 418 I'm a teapot
Date: Wed, 25 Feb 2015 07:08:27 GMT
Connection: keep-alive
Transfer-Encoding: chunked

The actual node.js code used to get this response was:

require('http').createServer(function(q,s) {
    s.writeHead(418);
    s.end();
}).listen(80);

http.Error(w, http.StatusText(418), 418)

Within native libraries, starting from Python 3.9 (Details @Ross)

Google does it.

Try clicking on the teapot, or tilting your mobile device.