The file should be named according to the language it was written. The name shouldn't be abbreviated too far (Javascript.js instead of JS.js) and it shouldn't be abbreviated to little (CSS.css instead of Cascading Style Sheets.css). Additionally, hyphens should be used instead of spaces (Objective-C.m instead of Objective C.m).
The code in the file should be the simplest or most straightforward way to print/display/show Hello World exactly once in the language used. Input from the user shouldn't be asked for where possible and a newline should only be printed when necessary.
update_list.py (Python 3.6+) should be run to automatically update the language list.