A lingua franca is a language that is used as a common means of communication between people with different mother tongues.
The term lingua franca is used in particular, but not necessary, when none (or only a minority) of the speakers concerned have the language in question as their mother tongue,

The expression has nothing to do with France as it was in Napoleonic times but the origin comes from the Italian, namely "lingua franca" or "Frankish language"
and the Franks were a collection of Germanic peoples, whose name was first mentioned in 3rd century Roman sources, associated with tribes on the Lower and Middle Rhine in the 3rd century AD, on the edge of the Roman Empire. Later the term is associated with Romanized Germanic dynasties within the collapsing Roman Empire.
Francia, also called the Kingdom of the Franks (Latin: Regnum Francorum), or Frankish Empire was the largest post-Roman Barbarian kingdom in Western Europe. It was ruled by the Franks during Late Antiquity and the Early Middle Ages.

The core Frankish territories inside the Roman empire were close to the Rhine and Maas rivers in the north. After a period where small kingdoms inter-acted with the remaining Gallo-Roman institutions to their south, a single kingdom uniting them was founded by Clovis I who was crowned King of the Franks in 496. Under the nearly continuous campaigns of Pepin of Herstal, Charles Martel, Pepin the Short, Charlemagne, and Louis the Pious—father, son, grandson, great-grandson and great-great-grandson—the greatest expansion of the Frankish empire was secured by the early 9th century.

The tradition of dividing patrimonies among brothers meant that the Frankish realm was ruled, nominally, as one polity subdivided into several regna (kingdoms or subkingdoms). The geography and number of subkingdoms varied over time, but the particular term Francia came generally to refer to just one regnum, that of Austrasia, centred on the Rhine and Meuse in northern Europe. Even so, sometimes the term was used as well to encompass Neustria north of the Loire and west of the Seine.

Francia is regarded as the common predecessor of the modern states of France and Germany. After the Treaty of Verdun in 843, West Francia became the predecessor of France, and East Francia became that of Germany. Most Frankish Kings were buried in the Basilica of St Denis near Paris. Francia was among the last surviving Germanic kingdoms from the Migration Period era until its partitioning in 843.

The singular use of the name Francia eventually shifted towards Paris, and settled on the region of the Seine basin surrounding Paris, which still today bears the name Île-de-France and gave its name to the entire Kingdom of France. The most prominent other places named after the Franks are the region of Franconia, the city of Frankfurt, and Frankenstein Castle.