There is actually an entire cassette dedicated to explaining how the parasites would discern between different dialects and overlapping words/phrases.But seriously Skull Face is a terrible villain and his plan doesn't stand up to any scrutiny. A lot of the major languages spoken in the world are very similar in ways the parasites couldn't possibly be discerning enough to work properly, also many languages incorporate common words from other languages he would've killed the entire human race after everything was said and done.
Also his face doesn't resemble a skull at all.
You probably fell asleep listening to it because it was Code Talker and he can be boring as fuck.