I previously said,
hardcoded SaveAs menu.
Good news: there is at least a workaround in the Save As menu. It turns out that wasn’t as hardcoded as I thought.
If you go to %AppData%\Notepad++\langs.xml, and edit it (following config-file-editing recommendations), adding<Language name="Fake" ext="fak fake false"/>
… then save and restart, then File > Save As gets a new entry:
It’s poorly named as “Normal text file” again, but it has a different list of extensions (and the original “Normal text file (*.txt)” at the beginning is still there, so you lose nothing).
By my experimentation, where you put that line in langs.xml will determine where in the SaveAs list it goes, so if you use it often, put it near the top. For example, if I move it from the end of the list to right after the normal “Normal”:<Languages> <Language name="normal" ext="txt"/> <Language name="Fake" ext="fak fake false"/> ...
… then it will go right after the normal-“Normal” in the list:
So you would miss the name of the UDL in the Save As list, but that would at least have that extension list, and when you select that new entry, it will default to .fak appended to your filename. If you have your UDL set up to be automatically associated with .fak (that is, the user extensions box in the UDL setup shows as fak fake false to match my langs.xml entry), then once you save the file as something.fak, Notepad++ immediately changes the selected Langauge to your UDL.
Unfortunately, that doesn’t put it in the Settings > Preferences > New Document > Default language group, but it is a step in the right direction.