All cars since 1996 have seatbelts that lock somehow (pre-crash).
In that car, the belts lock at the latchplate (the part you buckle in). If you buckle the belt, then pull up on the lap portion, you'll find that it doesn't move. Those belts are fine--in fact, often preferable--for installing car seats. The shoulder portion won't lock, but that's ok.
If you find that the car seat loosens once it's installed, you might need to flip the latchplate over (i.e., buckle it upside-down) to keep it tight. The belt locks when the two parts of the belt are parallel to each other, and sometimes a car seat makes the latchplate sit at a funny angle. Flipping it will solve that problem.
For the record, if you ever do encounter a car (pre-1996) with belts that truly do not lock, there are car seats that have built-in lockoffs, or you can use a locking clip. (A locking clip should not be used with locking latchplates.)