The historical context of Harold Pinter's Mountain Language is the treatment of the Kurdish people in Turkey by the authorities. Though the play is not intended as a parable or some kind of allegory on the treatment of the Kurds by the Turkish authorities, their plight nonetheless provided the inspiration for Pinter's writing of the play.
The mountain people depicted in the play are similar to the Kurds in that their language and culture are repressed. As the sergeant says when interrogating the young woman,
Your language is dead. It is forbidden. It is not permitted to speak your mountain language in this place.
Language is a very important component of a culture, and so its suppression is often fatal to the continued survival of a cultural tradition. The authorities depicted in the novel clearly see the language of the mountain people and the culture of which it forms such a vital element as containing within it the seeds of dissidence.
The very act of speaking this language is perceived as a threat to the stability of the regime. Hence the insistence of the prison guard that the young woman speaks what he calls “the language of the capital.” In his eyes, this is the language of order, stability, officialdom. Any language that the authorities cannot understand can easily become a source of subversion, a means of disseminating information prejudicial to the good order of the state.
In the hands of the authorities, the official state language is used as a weapon against the mountain people, a way of dehumanizing them, of keeping them in a position of permanent subjection. If the mountain people wish to speak in their own language, they must obtain the permission of the authorities, a sure sign that their culture is being repressed. It is noticeable in this regard that, towards the end of the play, when the prisoner and his mother speak in their own language, it's only because the guard has given them permission:
Oh, I forgot to tell you. They've changed the rules. She can speak. She can speak in her own language. Until further notice.