Blockquotes in at least one language
Here's an example that works both in Python and C++
#include <iostream> /* """ */ int main() { std::cout << "Hello World!\n"; } /* """ print("Hello World!") # */ Luis MendoLuis Mendo pitched what I think is by far the easiest solution, which is to use comments.
You look for one language that has block commenting and another language where regular syntax in the first is commenting syntax in the second.
Even easier is two languages with different block commenting styles that are interchangeably correct syntax, but I couldn't be bothered to check.
Check it out in Python 3.5 and C++