Enderman idle noises are actually sound clips of various people saying "Hi," "Hey," "What's up?" "Hello," and "Thank you!" when a player kills the Ender Dragon, all of which are human voices being distorted. On one of the sounds, you can hear a "Look for the eye". Creepy, right?
Do Endermen have a language?
Yep! Endermen, before the Ender Dragon, had a pretty unified system of speech, as trade + business between end islands had pretty much homogenized (?) endspeak and endscript.6 Aug 2018
Who made enderman language?
Tynker
What Enderman are really saying?
On November 13th, 2011 endermen received their own audio scheme, most notably a long moan overlaid with static for when they are triggered hostile by the player. The noises are actually a person saying "Hi" and "Look for the eye" and "What's up" backward and distorted.