I have created a fairly simple user-defined language in Notepad++, and given it an associated extension. But, when I open files I have named with that extension, nothing happens
This has been fixed, so I guess the whole question could be closed or deleted. I'm not sure in which version they fixed it, but in the latest v6.5.1 everything works as intended.
Language > Define your language...
Ext.
text box add your desired extension with no leading "." and space separated if you have multiple extensions.userDefineLang.xml
usually stored in your %APPDATA%\Notepad++\
folder.Here is a clear guide for how to create your own Notepad++ syntax definition for a new language.