Perl is preinstalled on pretty much every Linux and MacOS since time immemorial, and it's pretty much guaranteed to stay preinstalled for the foreseeable future.
Python3 is up and coming, and eventually it will get there, but old machines must die before it can reach ubiquity.
It will be a long time before it makes sense to use python where longevity/portability matter.
Python breaks backwards compatibility with old scripts every six months or so. In practice, this means you have to port scripts every time you switch machines. Virtual environments sometimes address this issue, but they are hit or miss.
In contrast, perl scripts from 1999 often work unchanged on clean 2022 OS installs.
Yes, I would consider awk and sh lingua francae (?) alongside Perl – but in slightly different contexts, namely low-complexity jobs.
I find Perl slightly more suitable once things get a little complicated, mainly because it's footgun:power ratio is better than awk (few footguns, but also very low power) and sh (many footguns, not that powerful).
I have come across plenty of boxen where bc was not installed, so I don't put that in that bucket.
And yes, if I want to describe text changes in a highly portable way, I will use sed (or ed, depending on context) rather than, say, unified diffs. I could accept an argument that unified diffs are also a lingua franca for more complicated changes, but they are harder to write.
Then what would, in your opinion, make it "lingua franca"?
I took it to mean "so common that it's available almost anywhere" (which is consistent with the context of the comment).
I've found it extremely useful to rely in tools such as the ones you just listed. Specially because they are often present even in light vms were perl might not.
