Robots.txt and language exclusion
Drupal by default comes with robots.txt file. You can find the file in Drupal Root.

Yes, you can go ahead and change. Below code also should work.

Disallow: /fr/


But, if you use 

Disallow: /fr/*


As per wikipedia


  Universal * match The Robot Exclusion Standard does not mention
  anything about the * character in the Disallow: statement. Some
  crawlers like Googlebot and Slurp recognize strings containing *,
  while MSNbot and Teoma interpret it in different ways.