Lingua Franca. Definition: A language mutually understood and commonly used in trade by people who have different native languages Example: ENGLISH Application: English, being the Lingua Franca, is the most important language in the world because the majority of trade conducted is done in English.  .efinition: A language mutually understood and commonly used in trade by people who have different native languages Example: ENGLISH Application: English, being the Lingua Franca, is the most important language in the world because the majority of trade conducted is done in English.   .