This sounds pretty similar to the definition of a species.

If two organisms of appropriate gender can't reproduce with each other, then they probably aren't the same species.

If code from two samples can't be interspersed, then it's probably not the same language.

Under that definition, Perl and Python have finally merged, given Perl 6's Inline::Python. Peace at last!

I'd probably go with a definition more like "mutual intelligibility" like for natural languages, but that doesn't seem right either. Sometimes I have to turn on the subtitles for British TV.

Yeah, under that definition everything that supports FFI has merged into a massive unholy mess with C. I guess if it can't be "natively" compiled/interpreted as the same language without inlining or including, I'd say it's a different language.