A natural, national language used outside its national boundaries by other peoples can serve as an international language. Latin, for instance, was a universal language in Europe during the Middle Ages and the Renaissance. French was once known as the universal language of diplomacy, and English today is often said to fill such a role in world commerce.
A modified, greatly simplified form of an existing national language has also been suggested as a possibility for an international language. One noteworthy example is Latino Sine Flexione ("Latin without inflection"), the brainchild of Giuseppe Peano, an Italian mathematician of the early 20th cent. Also sometimes confusingly called Interlingua (it is not related to the more widely known artificial language of that name), it is essentially a very simplified form of Latin. It too failed to gain widespread adoption, partly because its vocabulary was too extensive for the average person to master.
More recently, Basic English, a dramatically simplified form of English, has been proposed as an international secondary tongue. Developed between 1925 and 1932 by the English scholar C. K. Ogden, it has a reduced vocabulary of 850 words and an uncomplicated grammar. The vocabulary is composed of 600 nouns, 150 adjectives, and 100 other words that include verbs, adverbs, prepositions, and pronouns. Basic English has several features that make it suitable as an international auxiliary tongue. It is easy to learn and adequate for satisfactory communication; in addition, it is a simplified form of a widely used, and therefore very familiar, world language.