Another point is that a lingua franca is a common tongue for communication between people with different mother-tongues: not a replacement for native languages. For example in East & Central Africa Swahili is used for cross-communication, but individual ethnic groups maintain their cultural identities by holding onto their own languages. In this sense, I don't think you could easily find a developer who would struggle to understand anything (simple enough) written in Python, even though they don't use it on a day-to-day basis, so I think languages like Ruby/Python are already sufficient lingua francas for today's developers.

One more thing, maths notation is ridiculously diverse and overloaded and is in no way entirely uniform -- or even close. This is a problem when the disparity exists within a branch of maths, but an advantage when it's between very different areas, such as Analysis and Combinatorics.