Steps to reproduce:

1. Create an Animator Controller component for an object
2. In the Animator window, create a state for the controller
3. Select the state in the Animator window
4. Click on the "Add Behaviour" button
5. Click "New Script"

Notice that there's no programming language option like in Add Component. Furthermore, creating js externally doesn't help, since it's impossible to edit Script field and therefore to drop js instead of already added C# script.

Reproduced with: 5.2.4f1, 5.3.6p1, 5.4.0f3, 5.5.0a4