Crouching is a fairly unnatural position, and is generally done for a short time. We crouch to get as low down as possible or to take up as little space as possible. We might crouch down for a short while to talk to a very young child, for example. We might also crouch down to hide. We tend to crouch with hunched shoulders, especially if we are trying to hide or are forced to crouch in restricted spaces.

Squatting, by contrast, is a more natural and healthy position. It's simply a way of 'sitting' on our haunches when there is no chair available. We squat with our upper body upright and shoulders straight, and, with practice, you can squat for a relatively long time. In many non-western cultures, squatting may be a normal way to sit while working or socialising.

"crouch"
Adopt a position where the knees are bent and the upper body is brought forward and down, typically in order to avoid detection or to defend oneself.

@Sean
I agree with you.  They are not always interchangeable.