It would not be difficult, at all.
The problem is you are not separating reporting of and making statements of fact with statements of opinion. We're not talking about "I think your opinion is wrong" when talking about this kind of monitoring.
Macron is not targeting "I think Macron is stupid and his policies are idiotic," he's targeting "Macron eats babies and has sex with puppies, which is why he is evil." A US example would be the one about Clinton and a business being linked to a child abduction and pornography ring. While, yes, that is pretty defamatory, the claim was that this was being run in the basement of the pizza restaurant. That business's building did not have a basement level. And yet that "story" was spread very widely, and even by high-level government and campaign officials.
The first clause of the second statement is presented as fact, and can easily be demonstrated to be true or false. Or, at the very least if it's a case of "you can't prove the negative," it can be determined, quite easily, that the person making that claim has met or has not met an objective standard of evidence before offering that claim.
Obviously, there would be a lot of discussion over where the burden of proof lies (you have to have proof before making a claim vs you have to be able to disprove to sanction for a claim), and I think it would differ depending on the kind of statements being made, but the idea that facts are fungible and impossible to determine is a concept that, basically, people who find actual facts inconvenient for their agenda like to claim, but has no logical basis.
Note that my statement that it would not be difficult to create a standard is not necessarily an endorsement of the concept, just refuting the idea that the truth of facts is somehow impossible to determine.
One example of trying to enforce standards of reliability or honesty, and not just having an excuse to crush any critical or dissenting opinions is Canada's Radio Act. While not all-encompassing, it does apply certain standards:
It is against the law for any organization that is licensed as a broadcaster to broadcast false or misleading news. This was actually, originally, in Canada's criminal code and was used to prosecute a Holocaust denier, Ernst Zündel, in the early 1980s, but the Canadian Supreme Court ruled that it was too much of an infringement on free expression rights. The ruling was that these requirements were to be used as regulations, instead.
Is Fox News banned in Canada? | PunditFact - PolitiFact
(Note, the reason I referenced the above link is because it contained the passage where they describe the previous law and use against the Holocaust denier).
The previous Prime Minister in Canada wanted to get rid of these regulatory requirements, completely, in 2011 but that initiative was rejected and the regulations stand.
CRTC ditches bid to allow fake news - The Globe and Mail