Scientists are well aware that Plotties have their own language and numeral system. It is believed they may even have different dialects. However, despite the advances in Plottie language research they haven't been able to translate their language yet.
Plotties speak in a much lower frequency compared to humans, so their speech can only be perceived as high-pitched noises and squeaks, similar to chirping.
Although humans and Plotties can't talk to each other, they've been able to pick up certain gestures, reactions and mannerisms. Overtime, Plotties have become domesticated enough to try and emulate them. A human should be able to have basic communication with a Plottie through the use of body language.