Scientists (among other professionals) from different fields and backgrounds tend to treat these two concepts as synonyms. But I always saw an essential difference between a "lingua franca" and a "working language". For me, a "lingua franca" was built with portions of different languages but with no specific identity, structure, or rules. For instance, centuries ago, merchants from different nations, used to communicate with each other by combining portions of their different languages and creating a lingua franca with no specific semantics or grammar, that changed in space and time depending on who used it and where it was used. On the other hand, I see a "working language" (like English, Latin, German, etc) as one with its own unique identity, with grammatical and semantic rules that don´t change significantly in space/time. An existing language that happens to be chosen by a group of people (for different reasons) in order to communicate with each other, taking advantage of having a given specific (pre-established) structure, grammar, and semantics.
I often suspect I was wrong about this, but I'm not sure. Can somebody bring some light and knowledge to clarify this?