Short answer, no, if the decal is opaque enough. If in doubt, paint the foil white.
Better yet, download a flag into your photo editor. Make a backside copy. Insert this into a word document as clipart. resize to what you need. Print at the highest resolution onto standard white copy paper and allow the inks to dry several hours at least. Seal the inks with Microscale decal film/decal saver. Allow this to dry. Using a straightedge and a #11, cut the flag out. Crease in the middle. Take some diluted white glue and lightly coat the inside. Wait a couple minutes as the paper absorbs the water and softens. Carefully fold along the You may need to touch crease and roll out any excess. Put furls as you want or drape it on your vehicle. Allow to dry and you have a very realistic flag. You may need to touch up the edges with the appropriate colors.