The flaw in this theory was exposed when I couldn't have a conversation with any of my monolingual Italian relatives until I had learned their language in adult life.
For modern language teachers, this story is no doubt galling. They are already fighting a hard enough battle to get their subject taken seriously without natural allies failing in their duty of support. Indeed, the decision to make languages optional for 14-year-olds has made many despair that the Government has achieved the seemingly impossible: it has made language learning in the UK even less valued than it has historically been.
But is it really such a bad thing that children will not have to study languages beyond key stage 3? Those who say it is are on shaky ground if they make their case in strictly utilitarian terms. Were it not for my relatives, for purely practical purposes my father's reasoning would have been more persuasive. The problem is that English is indeed an international language, and there is no particular reason to speak any one other. I learned - or to be more accurate, studied - French at school, but the languages I needed in adult life were Spanish and Italian. The sole functional worth of five years of French is that I can irritate Parisians with my poor grasp of their language while on holiday.
The general value of learning to speak foreign tongues is much subtler. The Austrian philosopher Ludwig Wittgenstein said that the limits of his language were the limits of his world, and it is true that speaking a second language is like having access to another world. No other subject expands mental horizons in the same way.
However, this in itself does not clinch the case for compulsory language learning in key stage 4. All disciplines provide their own unique benefits, but not all are mandatory. For example, we are routinely told that those who fail to understand history are doomed to repeat its mistakes, but we don't seem too bothered that, on that basis, many of our children are doomed. And philosophy, which Radio 4's Today programme listeners recently overwhelmingly voted should be taught to children, is rare in schools, even at A and AS-level.
Perhaps most significantly, the chance to drop languages earlier is being counterbalanced by the opportunity to start learning them earlier.
Although there is some scepticism that the Government will meet its target of offering all primary school children modern languages by 2010, the number of primaries offering the subject is already 44 per cent, around twice as many as four years ago. So it is not at all obvious that over the span of compulsory education as a whole, language learning is being squeezed out.
What is more, we should not confuse the benefits of truly speaking a language with the benefits of passing a basic examination in it. I managed a grade B in my French O-level, but I never achieved any functional competence in French. Only in adult life, when I learned Spanish, did I get a feel for another language, and with it another way of thinking.
If we look at the wider context of language learning and its benefits, we can afford to be sanguine about its disappearance from the post-14 compulsory curriculum. Children are still getting exposed to foreign languages, and from a younger age. The keen and able will choose them as GCSE options, while the struggling and unenthusiastic are unlikely to benefit much from being made to persist for two years more.
And those, like myself, who find they need to learn another language in adult life can always do so.
Languages are always popular in adult education, where teachers find more motivated students who, despite the mythology of children as linguistic sponges, can progress quite quickly if they want to.
That's why I think it is possible to value the unique contribution modern languages make to our intellectual development and at the same time wave a content adieu to compulsory lessons post-14. Ce n'est pas si mal.
Julian Baggini is author of What's It All About? Philosophy and the meaning of Life (Granta).
Modern languages subject focus TES Teacher 22