lingua franca has 2 distinct synonyms spread across 1 of its 1 sense. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a common language used by speakers of different languages
“Koine is a dialect of ancient Greek that was the lingua franca of the empire of Alexander the Great and was widely spoken throughout the eastern Mediterranean area in Roman times”
Synonyms, antonyms and the broader and narrower relations for lingua franca come from Princeton University’s WordNet. Full sources and licences.