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Guarani, specifically the primary variety known as Paraguayan Guarani, is a South American language that belongs to the Tupi–Guarani branch of the Tupian language family. It is one of the official languages of Paraguay, where it is spoken by the majority of the population, and where half of the rural population are monolingual speakers of the language. Variants of the language are spoken by communities in neighboring countries including parts of northeastern Argentina, southeastern Bolivia and southwestern Brazil, and is a second official language of the Argentine province of Corrientes since 2004. Guarani is also one of the three official languages of Mercosur, alongside Spanish and Portuguese. Guarani is the most widely spoken Native American language and remains commonly used among the Paraguayan people and neighboring communities. This is unique among American languages; language shift towards European colonial languages has otherwise been a nearly universal phenomenon in the Western Hemisphere, but Paraguayans have maintained their traditional language while also adopting Spanish. Jesuit priest Antonio Ruiz de Montoya, who in 1639 published the first written grammar of Guarani in a book called Tesoro de la lengua guaraní, described it as a language "so copious and elegant that it can compete with the most famous [of languages]". The name "Guarani" is generally used for the official language of Paraguay. However, this is part of a dialect chain, most of whose components are also often called Guarani.