Another reason why people might struggle to get Syntax Highlighting working is because they don't have the appropriate syntax package installed. While some default syntax packages come pre-installed (like Swift, C, JS, CSS), others may not be available.
To solve this you can Cmd + Shift + P ? "install Extensions" and look for the language you want to add, say "Scala".
Find the suitable Syntax package, install it and reload. This will pick up the correct syntax for your files with the predefined extension, i.e. .scala
in this case.
On top of that you might want VS Code to treat all files with certain custom extensions as your preferred language of choice. Let's say you want to highlight all *.es
files as JavaScript, then just open "User Settings" (Cmd + Shift + P ? "User Settings") and configure your custom files association like so:
"files.associations": {
"*.es": "javascript"
},