can I substitute regular organic lentils in a recipe that calls for french green lentils?
Yes. There will be a slight flavor difference but its nearly the same as swapping cannellini beans for navy beans
You will get a slightly different consistency, but it's fine.
Regular lentils will break down a bit in the cooking process. It changes the outcome but not necessarily in a bad way. Just don't expect the same toothiness.
Also cooking times will be different so be sure to read the package. In hard water areas you may need to add a bit of baking soda.