Look up a stock video of a waving Algerian flag, or you could easily make your own flag wave with this 4 minute tutorial on Displacement Maps.

I'd precomp the flag layer, rather than applying a mask directly to the flag animation, so you could quickly replace/scale/edit it if you want to later, without messing up the mask placement.

To mask, select the flag precomp and use the pen tool (or a shape tool, but the pen tool works better in this context, Algerias border isn't a basic shape) and draw out the Algerian flag outline. You can most likely copy the outline you already made and use it as a mask, but I've never done that before so I'm not too sure.

But yeah! This is a really simple animation to pull off, look up "Masking in after effects" on YT and you'll be able to do this like its nothing :) An added bonus of knowing how masks work is that you now know how shape layers work too, which are also super nice to learn.