Complex sentence: Most JVMs, if not all, are written in low level languages such as C/C++ and assembler; what makes Squawk different is that Squawk's core is mostly written in Java.
Split into simpler sentences: Most JVMs, if not all, are written in low level languages such as C/C++ and assembler. What makes Squawk different from the other virtual machine is that Squawk's core is mostly written in Java.