How to set the file order (arrangement) permanently in Finder?
All folders except the Applications folder and its sub-folder will maintain your setting for Date Modified if so set in the 


  Arrange pull down menu






The Applications folder does not even has such a option.

However, if you're talking about the "Sort By" option, when you're in a folder press command+J to see the View options. 

Now go about setting the view options exactly how you'd like them to be. Once completed, click on the Use as Defaults button at bottom centre of the View options window. This will set your configuration as the default for all folders.