Drupal - Rule condition to check which language is used by the visitor?

Seemed like a useful thing to have.

I created a patch for Rules, adding this functionality. I posted it in the Rules issue queue. That seemed better than providing the code directly here, as this way you can still use it anyway that suits you, yet it may actually get into core Rules.

Also see this question, "How do I add a new condition in Rules 2?".