Ask visitor if he want to view page on his language
Just some ideas: I remember a WordPress multilanguage plugin setting that I had the option 


  Detect the language of the browser and redirect accordingly.


That must work similar for Drupal. So, I'ld guess you would try that browser language approach. D6 even had an own function for this language_from_browser. I bet there must be something in D7 for that as well.

Once you have the language you might conditionally display the modal window from that. Inside the modal then there'ld be a little form, maybe just two submit buttons "Yes language", "No language", something like that.

Inside your form submit function you may find a way to trigger the redirect to the chosen language. Finally you might want to save the user's answer in a $_SESSION variable and bind that as a condition for the modal display also.

Put all of that in a little custom module and let us know how you've done!