Since the courts have held that it is legal as First Amendment protected speech to burn a US Flag to make a point, there's no reason US Citizens can't make the same point by burning flags of foreign nations.
Granted, it's impolite, and so long as others refrain from desecration of the US flag, turn about is fair play. But when they deliberately insult or trivialize the US banner, then it's time for theirs to go up in flames.
As for those who've clothed themselves in such foreign banners, should that come to pass? Ish kabibble!
The physical act of burning a flag is the same act, regardless of which flag or whose flag it is. There are laws restricting outdoor burning, and laws against destroying someone else’s property, endangering others or inciting a riot. There are also laws against harassment and terroristic threats. All f those laws could apply to many instances of flag-burning.
But when you start passing laws based solely on the symbolic content of the act, the First Amendment becomes an issue. If I burn a US flag, a Nazi flag, a copy of the Bible, a copy of the Constitution and a copy of Where’s Waldo side by side, they’re all subject to the same penalties.
With some exceptions, of course. Time, place and manner count for a lot — Rallying Klan members have the right to burn a cross on private property as long as they obey environmental laws and have the permission of the property owner. But if they burn a cross on someone’s lawn, the nearly inescapable conclusion is that it’s a threat.