Here is the full list of all 20 Sanitarium achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Play the good melody for the first time
Speak with everyone
Find the children in the wood
Finish level 13
Finish level 12
Finish Level 11
Finish Level 10
Sort them
Finish Level 9
Finish Level 8
Finish Level 7
Find the sentence
Finish Level 6
Finish Level 5
Finish Level 4
Finish Level 3
Make dancing three friends
Finish Level 2
Win "Hide and seek"
Finish Level 1