Leyden and Language
Leyden was deeply interested in language and legend, and pursued linguistic study in a number of different languages. A preliminary survey of The Poetical Remains of the Late Dr. John Leyden (Morton) suggests that he knew Arabic, Hebrew, Greek, Latin, Tamil, Hindi, and probably Pashto. The fact that Leyden authored a "Comparative Dictionary of the Barma, Maláyu, and T'haí Languages" suggests that he probably had some competency with Burmese, Malay, and Thai as well. John Reith's biography brings the total list of languages up to forty-five--twenty-eight "Oriental" languages and seventeen European ones. (Reith, 379) According to Reith, these languages were:
Latin, Swedish, Canarese, Greek, Icelandic,Tamil, Italian, Sanskrit, Malayalma, Spanish, Zend, Mappila, French, Pehlvi, Maldivan, German, Persian, Prakrit, Hebrew, Pushtu (Pashto), Pali, Portuguese, Balochi, Jaghatai Turki, Aramaic, Hindostani (Hindi), Malay, Arabic, Dakhni, Barma (Burmese), Amharic (Ethiopian), Marathi, Rakheng, Gaelic, Gujerati (Gujarati), Siamese (Thai), Dutch, Bengali, Bugis, Flemish, Nuddya, Macasar, Danish-Norwegian, Telugu, and Armenian
The Tales of the Peries appears to have been translated by Leyden from Persian, which was one of his favorite languages. Morton, in his Poetical Remains, notes that while Leyden was ill in Seringapatam, he "made considerable progress in the Sanskrit language, and amused himself with translating tales form the Persic [Persian] and Hindostani." (Morton, xliii) In 1809, he was at work on two several Persian translations, as well as on what may have been an independent study that was never published: "I have also made considerable progress with a History of Modern Persian Literature, as well as with a series of dissertations on Oriental Languages."(Reith, 369)
The general shape of Leyden's linguistic inquiries appears to have been to trace out the historical development and family connections between the languages of central, south, and southeast Asia. He was deeply interested in discovering which languages were derivatives of others, and to tease out the connections between languages like Greek, Latin, Hebrew, and Aramaic and south Asian languages like the ancient language of Sanscrit. Scholars since the 17th century had noted similarities between languages being spoken in India and modern European languages, suggesting that there was some ancient similarity or common ancestor language that had broken down into many different Babel tongues. The term for this language, Indo-European, was coined by the English scholar Thomas Young in 1813, two years after Leyden's death.
References:
Morton, James. The Poetical Remains of the late Dr. John Leyden with Memoirs of His Life. London: Longman, Hurst, Rees, Orme, and Brown, 1819.
Reith, John. Life of Dr. John Leyden: Poet and Linguist. Galashiels: A. Walker & Son, 1923.