# Their tongues are deadly arrows

"Their words are like arrows that kill other people". Their speech is represented by their "tongues". 

# they speak deceit

The word "they" refers to "their tongues". 

# With their mouths they proclaim peace with their neighbors

"They speak, saying that they want peace with their neighbors"

# but with their hearts they lie in wait for them

"but what they really want is to destroy their neighbors"