Obviously, language does change rapidly -- but it also stays the same.
Taking the "language changes" ball and running with it (too far), you ought to reach the conclusion that there are as many variants of English as there are individuals speaking it. It's a little different for all of us. No wait... I don't speak the way I used to... Each of us actually speaks a few variants in a lifetime. But then, with nothing in common between us, there is no shared language to speak of at all.
That obviously isn't right.
Nor is it right to say languages don't change. But it might be more helpful to recognise that langauge diversifies -- and rather quickly. New technologies and social systems, new slangs, new pronunciations, more communication channels requiring more varieties of language styles to suit their purposes... But all the old ways of speaking still count as valid forms of English. The dictionaries couldn't cite any archaic English words otherwise!
It depends on the "level of magnification" you're using as to how static or dynamic you consider the language to be. Harper is talking about the grand scale: everybody in the English-speaking world can understand every other, across large expanses of space and time. Of course there will be cases where accents, slangs and so on cause difficulty, but they can be overcome by speaking slowly, choosing your words, dropping your jargon, writing it down... that sort of thing. (Changing your use of language to suit the hearer in order to be understood.)
But with a foreign language, no amount of putting yourself in the other person's shoes will help: you just don't speak the language. [Excepting, of course, that some times, with a closely related language, you will find a form of words that is recognised by the other. And there are yet more cases where you'll know what the other person is saying from his body langauge and gestures. All of this is fine detail, not relevant to the current argument.]
How we determine the point at which ancestral English speakers could communicate this freely with ancestral German speakers is another matter...