Am 23.04.17 um 10:50 schrieb Benedikt Vitecek:
> 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.


as I expected, babel does have an option to enable the shorthands 
already in the preamble:

[KeepShorthandsActive]

however, as I also expected that doesn't  work in all cases as  packages 
loaded afterwards may have a problem with that.


So

   \usepackage{xparse}

   \usepackage
     [KeepShorthandsActive]
     {babel}

solves your problem but if babel is loaded before xparse, that will break.

On the other hand, what might be enough in your case is simply changing 
the catcode of < and > before making your definition since we know that 
it will be active by the time of begin document (ie when your definition 
is used):


   \documentclass[spanish]{scrartcl}

   \usepackage{xparse}

   \usepackage{babel}

\catcode`\< = 13
\catcode`\> = 13

   \NewDocumentCommand \Something { d<> m }
     {
       Optional: #1 \\
       Mandatory: #2
     }
  \catcode`\< = 12
  \catcode`\> = 12


   \begin{document}
   \Something<Hello>{World}
   \end{document}

Of course that only works because we know that spanish will activate 
them, so this is not robust in a general way.

Guess the correct answer is drawingboard and make xparse babel aware and 
long term a better shorthand interface / integration

frank