When people have varying levels of capability with languages, they’ll switch to whatever is the lowest common denominator — the language that the group can best communicate in. This tended to be English, even amongst a bunch of native speakers of a common foreign language.
Moreover, this is context dependent: when talking about technical matters (especially computing), the Lingua Franca (pun intended) is English. You’ll hear “locals” switch to either mixed or pure English, even if they’re not great at it. Science, aviation, etc… is the same.
Before English it was French that had this role, and before then it was Latin and Greek.
The thing is, when the whole world speaks one common language like Latin or English, this is a tiny bit sad for some Gaelic tribe that got wiped out culturally, but incredibly valuable for everybody everywhere. International commerce becomes practical. Students can study overseas, spreading ideas further and wider. Books have a bigger market, attracting smarter and better authors. There’s a bigger pool of talented authors to begin with, some of which write educational textbooks of exceptional sparkling quality. These all compound to create a more educated, vibrant, and varied culture… because of, not despite the single language.
