Maribor, the picturesque second city of Slovenia, is basking in its status (which it shares with Guimarães in Portugal) as European Capital of Culture 2012.

Like Glasgow in 1990, which boasted Peter Brook's spectacular production of the epic Mahabharata, the Slovene city opened its flagship Borštnik Theatre Festival with a large-scale production by one of Europe's most famous dramatists. However, whether acclaimed German theatre maker Heiner Goebbels's show, When The Mountain Changed Its Clothing, will enjoy the legendary status of Brook's production is a moot point.

The piece is a work of music theatre performed in the splendid main auditorium of Maribor's National Theatre by a cast of 40 girls and young women aged 10 to 20 from the local choir, Carmina Slovenica. It combines dissociated texts by authors including Jean-Jacques Rousseau, Gertrude Stein and Ian McEwan with moments of song and choreography.

Exploring the uncertainties embedded in the transition from girlhood to womanhood, it begins with the provocative assertion by French writer Alain Robbe-Grillet that "young girls dream of- the knife and of blood". The work takes us from archetypical images of girlhood – such as each cast member carrying a soft toy – to ruminations upon living in the shadow of nuclear weapons.

As Goebbels says, he and his Slovene co-director, music conductor Karmina Šilec, have created a sort of patchwork quilt of script, movement and music. Unfortunately, the outcome of this assemblage is a theatre piece which shifts constantly between peaks and troughs. Although the young cast speak in wonderfully clear English, the surprise at hearing such adult concerns in the mouths of such young girls soon wears off. As the potency of the spoken sections of the play recedes, one is left marvelling at the choral power of the cast – at one moment, towards the end of the show, they sing polyphonically with a visceral impact one typically associates with far more experienced performers.

I was positively excited to see the Ljubljana City Theatre's Slovene-language presentation of surely one of the finest plays to emerge in Scottish theatre in the last 15 years, Zinnie Harris's Further Than The Furthest Thing. Played in the small studio theatre in the basement of the National Theatre, director Tijana Zinajic's production is dominated visually by a picture of Queen Elizabeth II and a crude painting of the sea meeting the remote south Atlantic island of Tristan de Cunha (where the play is famously set).

In both the space available to the actors and in design, this staging compares badly with both Irina Brown's memorable premiere (for the Tron Theatre, Glasgow and the National Theatre, London) in 2000 and James Brining's fine production at Dundee Rep earlier this year. Nevertheless, despite this, and a rather static first half, Zinajic's staging does ultimately reflect much of the power of Harris's imaginative tragedy of humanity assailed by isolation, starvation and deception.

That it does so is due in no small part to Alenka Klabus Vesel's translation. One of the most remarkable aspects of Harris's play is the manner in which it captures the distinct and arcane dialect of the Tristanians. One need not speak Slovene to realise that Vesel has done a remarkable job of inventing a parallel version of her own language, just as Harris did with English; this is all the more remarkable given the huge differences in syntax and grammar between English and Slovene.

The actors are observably comfortable with the rhythms of the Slovene script, none more than Judita Zidar, who gives a superb performance as the Tristan Islanders' matriarch, Mill Lavarello; a role which was defined brilliantly by Paola Dionisotti in the premiere production, and which delivered Ann Louise Ross the Best Actress award at this year's Critics' Awards for Theatre in Scotland.

Like Dionisotti and Ross before her, Zidar achieves a tremendous combination of innocence, anguish, tenacity, defiance and bleak comedy. By the time she, like the chorus of an Attic tragedy, recalls the horror of the starving islanders drawing lots to decide who would eat and who would not, I, as a member of the Festival's prize-giving jury, was sorry that the production was not one of the 12 Slovene shows selected in the competition.

Elsewhere in the opening days of the Festival, Slovene theatre was foregrounding tiresome, post-modern, faux-radical tendencies which I had seen on my last visit to Maribor in 2010. In Mandicmachine, for instance, high-profile actor Marko Mandic rips through his greatest hits, with roles by an array of authors including Chekhov, Sarah Kane and Howard Barker.

For more than an hour-and-a-half, Mandic assaults his audience. Whether he is screaming into a microphone, getting his kit off (a signature move), simulating masturbation or dragging often unwilling audience members onto the stage, this contemptible actor gives a performance as hollow, gratuitous and adolescent as I have ever seen. That Mandicmachine has been selected in the Slovenian showcase here in Maribor, while Further Than The Furthest Thing has not, is a tragedy in itself.