WHEN I arrived at school in England, my English was strange because I'd learned it from books. I was sent to Coventry after I explained that gold was made of carats. "Carrots," my classmates cried, whooping with glee. "She says gold is made of carrots," they cackled. I didn't know what "being sent to Coventry" entailed as the phrase didn't figure in Rider Haggard or the Hornblower books, which were my passion. But when I found myself in it, I rapidly learned to speak like a native, to cry out "Vamoose!" and say "No fear", and "Hard cheese", and a whole huge vocabulary previously unknown.

In contemporary Britain new forms of exclusion are often introduced or suggested on grounds of fluency. In June 2010, the secretary of interior, Theresa May, said: "I believe being able to speak English should be a prerequisite for anyone who wants to settle here. The new English requirement for spouses will help promote integration, remove cultural barriers and protect public services."

A language test has been established for British citizenship since 2002 (the same kind of exam exists in the US, while vast cities within cities in Texas and California are dominated by Spanish or Korean or Japanese speakers, many of them flourishing communities).

This demand for proficiency in English has become coupled here with a different anxiety about the country's increasing monolingualism, as native English speakers are less and less eager or able to acquire a second language, and enrolment in foreign language courses is falling.

Politicians are seeing things back to front (not for the first time). My own experience of early contact with other languages has made me realise several things. First, it is possible to speak another language fluently and yet make continual mistakes in it - mistakes of word order and phrasing, register and weight of terms used, and numerous other pitfalls. Second, languages are mobile, capricious, and have a voracious appetite. The front page of a newspaper will contain a huge percentage of new turns of phrase and vocabulary - Le Monde or La Repubblica, after only a few months' absence from France or Italy, will take a lot of patient decipherment. And that is at the top end of the periodicals market. When it comes to the tabloids or comics, the popular bandes dessinees and fumetti, their vernaculars become completely opaque. I feel very strongly that the ideal shouldn't be mastery of another language, because that's an unachievable goal and holding it up as the aim just makes students feel hopeless. Good-enough English, good-enough French, good-enough Arabic is quite enough - to adapt the wise and humane British psychologist DW Winnicott's term, "good-enough mothering". We should emphasise the pleasures of languages, rather than the need for complete competence.

I now teach creative writing and many of my students are non-native English speakers. Although they speak English well, you couldn't say they are fluent. Most of them have more than a smattering, but the task of writing, and writing imaginatively in English demands that they push themselves hard to capture on the page what they are seeing in their mind's eye. But the results are often strikingly vivid.

There are gains from not knowing a language as one's mother tongue - as Samuel Beckett realised when he set aside English and chose to write in French. Unfamiliarity helps. In my current MA class, one of the most gifted writers is Mexican. In Abu Dhabi, where I taught undergraduates, mother tongues included Spanish, Korean, Arabic, and Kutchi, a language I had not heard of before. The Kutchi and Arabic speakers wrote - in English - some of the most sensitive work produced by the class. Mistakes are easily fixed, usually.

Perfection of linguistic fluency isn't of prime importance for expressive power.

As a child of a woman who didn't speak English to begin with, I see value in making the crossing from one language to another without fear or inhibition, and, above all, for not minding making mistakes. Languages matter, but smatterings will do.

By arrangement with the Guardian