> What happens if you do your definition \AtBeginDocument?  Does babel
> turn on shorthands too late for that?

I’ve tried it before and sadly it doesn’t work and produces the same output:

  \documentclass[spanish]{scrartcl}
  \usepackage{babel}
  \usepackage{xparse}
  \ExplSyntaxOn
  \AtBeginDocument{
  \NewDocumentCommand \Something { d<> m }
    {
      Optional: #1 \\
      Mandatory: #2 \\
      Other: 
    }
  }
  \ExplSyntaxOff
  \begin{document}
  \Something<Hello>{World}
  \end{document}

> Here at some point the
> command you define must grab an argument delimited by ">".  Given how
> TeX works, we need to know what category code this ">" will have.  In
> principle it could be conceivable to do that depending on the category
> code of the "<" that started the argument, or depending on the current
> category code of ">"


I am glad that this is the first (maybe second) time I have to deal with category codes, 
it seems to be quite complicated.

I think what I might be able to do locally is to check if the „m“ argument grabs the token
„<„ (which I should be able to test with \str_if_eq:nn {#2} { < } ), define a macro which collects the 
next tokens until it encounters a „>“ (again tested with \str_if_eq:nn) and then maybe call my
main command again.

This is only a rough idea and I would need to figure out how to implement it, but it could solve (at least
for me) the category code problem. 

Ben