The Languages menu on the menu-bar includes the list of built-in languages, and below those are a separator followed by Define Your Language... and a list of any UDL that have been already defined.

For these languages, Notepad++ supports syntax highlighting (customizable), syntax folding, auto-completion (customizable), function list (customizable via PCRE in xml file). If your beloved language ...

Notepad++ is a free (as in "free speech" and also as in "free beer") source code editor and Notepad replacement that supports several languages. Running in the MS Windows environment, its use is ...