What does it mean to “know” a language?

I’d like to share with you an excerpt from the book Interpreting for International Conferences by Danica Seleskovitch. I recently reviewed this book for my medical-interpreting course and I bookmarked things on every single page. I think it should be -if it isn’t already- mandatory reading for all interpreters.
I found it to be not only full of useful information but also very inspirational. I have always admired interpreters and their ability to switch “from one mental universe to another”, and when some years ago I wanted to learn more about how they do it, how their mind performs such a complex task, I inquired in an online T&I forum about book recommendations on this subject. This book by Seleskovitch was the one that was most highly recommended to me by many colleagues. Among the numerous things that are worth quoting, I’m copying here a passage that discusses what it means to “know” a language. How many of us have been asked “How many languages do you speak?” when we told someone we are translators? Of course the number of languages is somewhat -though not entirely- irrelevant to how good a translator is, but I have long stopped trying to give long explanations about this, it’s just easier to answer with a number, or give a vague answer like “a few”. However, my answer always varies; first of all because I’m never happy with it, I find it inaccurate, so I keep changing it (inaccurate because… do I include the languages I’m currently learning and which I can speak moderately well? Is “moderately well” good enough?); and second, because my level of knowledge of each language has changed with time. French used to be my second language; now I think it is my fourth. I used to speak Dutch fairly well but now I can barely utter a grammatically correct sentence without first thinking about it for 30 seconds. Should I include Dutch in the languages I speak? Do I call it a “passive language”, forcing the other person to ask what that is (assuming he’s not a linguist) and to think “gee, who cares, all I wanted was a number”? So now I just say “I speak a few”. If the other person insists, I say “I work with four, but I speak a few more”. And then I usually change the subject. I think I’ll print out this excerpt by Seleskovitch and carry copies of it with me. Here is what she had to say:

Nothing is more difficult than defining linguistic knowledge. What does it mean to “know” a language? A language is not a finite or clearly defined mass, which you either possess in its entirety or not at all. You do not “know” a language in the same way you know a theorem or poem by heart. You can only know it more or less thoroughly. Some speak two languages with perfect ease, yet have a very limited vocabulary in both. Conversely, philologists or authorities on theoretical linguistics, for example, who do not study languages for the purpose of speaking them, may have a very thorough knowledge of the languages they study, but would be unable to use them to communicate. Their knowledge is thus also limited.

Anyone who has to deal with the realities of today’s world has some knowledge, however minimal, of a foreign language, either because his job requires it or because he comes from a country where the language is not widely spoken beyond that country’s borders (the Dutch, the Swedes, the Poles and, increasingly, the French are finding themselves forced to learn another language). But neither the scholar with his literary or theoretical knowledge nor the expert with his specialized knowledge, nor the polyglot can be considered to have an exhaustive knowledge of the language, but merely a working knowledge. Acquiring a foreign language is so difficult that few specialists in linguistics are at the same time practicing linguists.

Seleskovitch, Danica. Interpreting for International Conferences. Pen and Booth, Arlington, VA 1998. (Translation and adaptation of L’interprète dans les conférences internationales – problèmes de langage et de communication, published by Minard, Paris 1968).