Approximately one in three residents of Greek Macedonia had Slavomacedonian as their mother tongue in 1912. In 1965, official reports estimated the number of Slavophones in just three prefectures (Florina, Kastoria, and Pella) to be 130-150,000. At the beginning of the 21st century, the "local" languages continue to be spoken in a large part of Northern Greece.

The eradication of this Slavic language was considered - and remains to this day - the major "national goal" in the region. The entire state apparatus, from the top of the administrative hierarchy to the simple police officer or teacher, was mobilized to achieve it. Contrary to what the dominant historiography claims, the period of the Metaxas dictatorship was not the only time of persecution of the "accursed foreign idiom," but the culmination of a policy that lasted for decades.

"The Forbidden Language" follows this policy of repression, intimidation, mockery, and "national" alarm in its historical development, from the beginning of the 20th century to the present day. A large number of unpublished documents were used in its writing, many of which are brought to light for the first time.