Specify language of Comment using Entity Translation?
Drupal natural way would be to show comments for different languages separately.

For example If you have a node in English and Spanish. Drupal natural way would be to show comments for en/es translation separately (attached to en/es content). 

If you are looking for language field you can use Language field module...


  This module creates a Language Field that combines Drupal 7's Field
  list options and Internal language list.
  
  This is used to simply collect language data.