Would love to be able to type in custom languages for homebrewed monsters rather than just the official ones. That is all.

You can under “Basic Information” using the “Language note Override” field.

Thanks for that. Got me closer to where I'd want it to be. Still isn't comma-delineated as other languages would be but at least it puts it on there.

You just have to add the commas.

Formatting-wise, that doesn't work. There is a space automatically inserted between official languages and the note override so at best you can achieve "Common , <language>" which is a small thing but still a thing (and drives my brain crazy...er). I can survive with it but it would be nice to be able to just add custom languages from the language options.