There's this question robots.txt vs language path prefix.
OP finds out that Drupal's default robots.txt doesn't actually cover his multi-language setup, which they configured to use path prefixes. An interesting question indeed. But is this really a Drupal-specific problem?
robots.txt is a customizable and generic file to communicate with crawlers, independently of what app, CMS or framework you are using. And multi-language is an optional feature of Drupal. Nothing the default robots.txt needs to cover from the start.
The same you wouldn't consider Drupal's default theme Bartik to cover all possible visual needs, you can't consider Drupal's default robots.txt to cover all possible path configurations. Talking about robots.txt in specific, there's http://www.robotstxt.org/robotstxt.html (and as SE network probably Webmasters) to get help configuring this generic file. It's also commented in the default robots.txt:
# For more information about the robots.txt standard, see: # http://www.robotstxt.org/robotstxt.html
robots.txt is SEO and I'd consider the mentioned question to be off-topic. Different it would be though if OP comes up with a question like "How to dynamically generate robots.txt to take language path prefixes into account" or similar, with a feature they built and a specific point where they are stuck. This can clearly considered to be on-topic.