robots.txt vs language path prefix
Whatever you are unhappy with, with Drupal's default robots.txt, delete it, and install RobotsTxt to provide an UI to let your SEO person edit the robots.txt according your needs.


  Use this module when you are running multiple Drupal sites from a
  single code base (multisite) and you need a different robots.txt file
  for each one. This module generates the robots.txt file dynamically
  and gives you the chance to edit it, on a per-site basis, from the web
  UI.




Look what I found. This duplicate issue on drupal.org robots.txt: add wildcarded paths for multilingual sites leads to this open issue Fix path matching in robots.txt, which actually could need your input.

But basically some commenters seem to recommend exactly what you came up with, wildcarding the path prefix.