Apparently, French intellectual life has been shaken in recent months by the publication of a book which suggests that contemporary French philosophers and intellectuals are basically a bunch of charlatans, who use grandiose vocabulary to disguise the vacuity of their ramblings. Why this accusation should come as a shock is anybody's guess, since English and American philosophers, with their traditionally stern faith in the meaningful and the empirically verifiable, have been saying that about the French for years now.

Still, nobody ever said that what's bad for philosophy has to be bad for the theatre - even the most rigorous logical positivist would have a hard time convincing anybody of the virtues of a theatre of verifiable propositional statements. Watching this week's two offerings in the French Theatre Season, you'd be hard put to it to extract any sense or meaning. But the audacity with which they avoid attempts to pin them down is one of the things that makes them intriguing.

Take Eugene Ionesco's The Chairs. On paper, it's no more than a flimsy exercise in Absurdity: an Old Man and an Old Woman prepare for a grand lecture, at which a celebrated Orator will deliver to the world the message that the Old Man has been working on for years. When the Orator arrives, to speak to a huge, invisible audience, represented by a crowd of empty chairs, the old couple kill themselves - this is the fulfilment of their lives' work. Unfortunately, the Orator can only grunt incomprehensibly, and, when he tries to write the message down, it reads as gibberish.

On stage at the Duke of York's, though, it's mind-blowing. The power of Simon McBurney's staging - a co-production between the Royal Court and Theatre de Complicite - is generated largely by Richard Briers and Geraldine McEwan. They look terrible: eyes red-rimmed, pipe- cleaner limbs taut and frail, bodies bent with weariness and despair - a pair of Addams family retainers. But they turn in performances of extraordinary wit and wildness.

Briers's acting sometimes has an offputtingly wheedling quality; here, though, with his caved-in face and an unaccustomed edge of impotent violence, it works to his advantage. As he flounders through a morass of self-justification towards the crowning moment of his life, the sum of all its efforts, he looks like a living warning of the dangers of, well, I'm not exactly sure what: existence, perhaps?

And McEwan is flawless. In the notes at the end of the script, Ionesco advises that, in order to keep up the rhythm and speed of the frenetic sequence when some 40 chairs are assembled on stage, the Old woman should be played by a young woman made up. Watching McEwan - born in 1932 - makes that idea look pretty stupid. The knee-trembler she enjoys with one of the invisible guests, her crazed tango with the invading mass of chairs, are masterpieces of raunchy energy.

They are just the focus, though, of a production almost perfectly judged in every department: the Quay Brothers' design, a ramshackle assemblage of worm-eaten wooden doors and battered chairs; the subtly shifting, occasionally dazzling lighting by Paul Anderson; Paul Arditti's sound design - thunderously slamming doors, a cacophony of bells and buzzers as the guests arrive. There are a couple of longueurs along the way, and the final scene - when the set dissolves to reveal the back wall of the theatre - strikes a false note. But overall, it leaves you feeling that there is some point to the Absurd after all.

The week's second French offering is Roberto Zucco, the last play by the late Bernard-Marie Koltes, best-known here for his Dans la solitude des champs de coton. Like that play, Roberto Zucco is set in a murky underworld, where social exclusion and philosophical disaffection seem to blur into one another. It follows the career of a young man jailed for killing his own father. Escaping, he murders his mother, has a brief encounter with a young girl, stabs a world-weary policeman (possibly out of kindness), goes out of his way to get himself beaten up, shoots an unarmed teenage boy through the back of the head, and finally gets banged up again.

As a piece of stagecraft, James Macdonald's production at the Other Place in Stratford is impressive. Jeremy Herbert's design places the action on a white strip running through the middle of the auditorium. This is painted with some sort of shadow-retaining fluorescent paint - a corpse left on stage for a few minutes leaves behind a dark impression of itself, like a bruise, or the blast shadows left by the Hiroshima bomb. The effect is of a world stained by the presence of people. Doorways are represented by bright red lines of light; a couple of scenes on Roberto's journey are represented by grainy, slow-motion video clips. It looks lovely.

Other aspects are less satisfactory. The translation is by Martin Crimp, who also did The Chairs. In the Ionesco, a combination of the queasily domestic with clumsy formality was very funny. Here, he seems to lapse into translatorese, that version of idiomatic English that seems to have been learnt from a dictionary. That may be a deliberate ploy, to remind you that this isn't the real world (I assume that's why Macdonald's actors have a thoroughly irrational variety of accents: Glasgwegian, Irish, Estuary, RP); but the heightened quality of the language sits uneasily with performances that are not so much heightened as broad.

It doesn't help that the play lapses into cliche - when a climactic stand-off with the police turns into a jokey routine about the public appetite for violence, it feels like a Natural Born Killers for the smart set. But it never quite succumbs to the categorising urge, and never quite loses its grip on your attention.

Meanwhile, the RSC continues the policy of presenting the best of foreign theatre with a production of Twelfth Night that's apparently a co-production with the National Theatre of Teletubbyland. Well, that's the impression you get from Anthony Ward's design, with its Dayglo primal colours and giant rubber props.

This bright and breezy appearance jars with Adrian Noble's rather sombre conception of the play. This Illyria is in some ways closer to the paranoid, hope-free zone that Roberto Zucco inhabits than to the dreamy space we tend to imagine, its inhabitants moved by neurotic, enervated passions - you would guess that Scott Handy's pale, weedy Orsino and Clare Holman's repressed Olivia both have to take something to help them sleep.

The awfulness of Viola's position, as she is cast adrift in this mad, self-absorbed, country, is made abundantly clear. A brief shower scene - "Cesario" peeking round the curtain at all the nude masculinity - establishes just how much strain and embarrassment is involved in these transvestite comedies. Later, commanded by Orsino to woo Olivia on his behalf, Helen Schlesinger collapses into heartbroken sobs. Aguecheek's challenge to a duel leaves her terrified and bewildered. To cap it all, Sebastian turns up and people suddenly start accusing her of doing all sorts of things she hasn't done; and Schlesinger registers genuine horror and bafflement - have they all gone mad? Has she gone mad herself?

So Noble gets the measure of the play's underlying sadness. The problem is, it isn't very funny. There's a scattering of sight gags - including the crotch-grabbing and priapus worship which is such a feature of RSC bawdy, and unheard of anywhere else - and a lovely sketch of myopic self- regard in Philip Voss's Malvolio. But Stephen Boxer's Feste seems gummed up with melancholy, and the normally excellent David Calder is a cardboard Toby Belch - dressing him up to look like Jimmy Edwards just reminds you of all the rumbustiousness and natural good humour that's missing. It's all rather depressing.

'The Chairs': Royal Court at the Duke of York's, WC2 (0171 565 5000), to 20 Dec. 'Roberto Zucco': Stratford Other Place; 'Twelfth Night': Stratford RST, (01789 295623); both in rep. Robert Butler is away.