Danton's Death, NT Olivier, London<br/>Lingua Franca, Finborough, London<br/>The Beauty Queen of Leenane, Young Vic, London

Saint-Just isn't best pleased. He reports glumly to his comrades how Danton has been acting "like a parody of Jupiter, roaring and shaking his locks" when put in the dock, accused of corruption by his arch rival Robespierre.

Cast in the title role in Danton's Death, Toby Stephens is certainly doing an awful lot of grandstanding – and it's nearer godawful than awesome.

This French Revolutionary drama by Georg Büchner (1813-37) is revived rarely in the UK, and one might conclude that's a mercy after this unengaging production. Many will have hoped for a more brilliant NT debut from Michael Grandage, a director who avidly admires historic German dramatists and has made Schiller, in particular, seem thrilling. Here though, you can only assume that Alec Newman's Saint-Just is depressed by the fits of hamminess brought on by his Reign of Terror.

Grandage's staging is undeniably beautiful to behold, played out in a wood-panelled chamber with hazy sunshine slanting through high windows (set by Christopher Oram, lit by Paule Constable). And, all right, yes, it does seem a shame that Stephens – so handsome in his high collar – has to be guillotined.

His epicurean Danton – pursuing lusty pleasures as much as égalité – is clearly meant to be a flawed but more likeable man than Elliot Levey's Robespierre, the cold-blooded puritan. Nonetheless, there's next to no debauchery or squalor beyond an unbuttoned waistcoat in this too-tidy production. Stephens's swaggering gusto (at least at this early point in the run) also seems stagey. The balance is hard to strike between genuinely impassioned and showboating political rhetoric.

Yet Büchner's more expressionist monologues are startlingly poetic, and the periwigged Levey is briefly intriguing: a lethal control freak, eyes darting feverishly in a rigid body. It's a shame, then, that Robespierre drops out of the picture. Büchner's play – the work of a fired-up fledgling dramatist – is bemusingly bitty, a flaw Grandage does his best to conceal with a climactic execution where it looks, gruesomely, as if heads really do roll.

Howard Brenton's filleted adaptation, to give credit where it's due, is clearer than the original. Your Jacobin might well object to the ditching of minor characters, but with so much of the acting in Grandage's ensemble falling flat, one can't help but wish that more of them had got the chop.

From the French Revolution, it's a sizeable jump to Italy just after the Second World War. Lingua Franca is a considerable coup for the go-getting Finborough Theatre, this being a new play by Peter Nichols – best known for Privates on Parade and A Day in the Death of Joe Egg.

Lingua Franca is, in fact, a seriocomic sequel to Privates. Here the same young male protagonist, the semi-autobiographical Steven Flowers, has moved on from National Service in Malaya to teaching at a language school in Florence.

Harbouring literary ambitions and reading E M Forster's A Room With a View, Chris New's Steven falls in love with the city. He's also swiftly befriended in the international staffroom. Ian Gelder's Jestin is an old English gent in a Forsterish linen suit. He becomes Steven's guide, introducing him to Renaissance masterworks and dispensing moral advice. The younger man, however, has a nihilist streak. He deceives Charlotte Randle's Peggy, a brittle, desperately lonely singleton. Steven lets her fantasise that they're romantic soulmates, when he's covertly humping the new German mistress, Natalie Walter's Heidi, a casual anti-Semite as well as a chronic floozy.

The language school manifestly serves as an analogy for the larger political picture, demonstrating how post-war hopes of international peace, love and understanding were naive. Tensions rise in the college, leading to a violent assault.

What's disappointing about director Michael Gieleta's premiere is that the staff's discussions about ethics sound authorially imposed. Yet Lingua Franca offers some witty exchanges and dramatic shocks. New's Steven simmers with frustration, and when it comes to torrid horniness, Enzo Cilenti outdoes him as the philandering boss. Rula Lenska, too, is splendid as a worldly Russian émigrée. If only the other women – Randle, Walter and Abigail McKern playing a brash Australian – didn't shout every line, as if English were our second language.

Singleton Peggy shares some psychological ground with Susan Lynch's Maureen, the poor, home-shackled protagonist of The Beauty Queen of Leenane. Martin McDonagh's instant classic from the 1990s, now revived by Joe Hill-Gibbins, is a dark but hilarious portrait of petty spite and savage cruelty in rural Ireland. Set in a mouldering hovel (grungy detailing by Ultz), the writing sits somewhere between Beckett, J M Synge and gothic horror. Rosaleen Linehan's Mag sits in her rocking chair like a malignant slug, making monstrous demands of her daughter and not half as senile as she pretends. David Ganly's Pato – Maureen's one-night stand and potential saviour – is by contrast profoundly gentle, an exquisite portrait of burly shyness.

Maybe Lynch needs to be more dangerously unstable. The plot twists started to seem increasingly corny on the night I attended, with a peculiarly hokey audience punctuating the performance with oohs and ahs.

'Danton's Death' (020-7452 3000) to 14 Oct; 'Lingua Franca' (020-7244 7439) to 7 Aug; 'The Beauty Queen of Leenane' (020-7922 2922) to 21 Aug

Next Week:

Kate Bassett genuflects before The Prince of Homburg, Heinrich von Kleist's German-history play