So we would have an international railway in France 
.
LoL
More ridiculous: the railway=rail I checked was named “Ligne de Paris à Strasbourg (LGV)“, both in France. A route may be of international importance, and for the importance of the rail we already have the tags we need, like highspeed, usage, gauge, …
I was talking about railways
Did you forget that the title of the thread refers to highways, not railways?
We have the TGV (Train à Grande Vitesse), so French being an international language, think than the motorways have to get the importance=AGV (Automobile à Grande Vitesse). Of course that would not fly in Italy, we’ll stick to Frecciarossa or Pendolino. 
Thanks for proving that “international” has little to do with “importance”.
In your original post you mention that “As a replacement for highway categories, some countries like France started using the proposed importance=* key.“, later, when @cquest reminds that France has never “started using the proposed” (…) as “a replacement for highway categories”, you say you’re speaking about railways.
So before starting a world-wide edit war (your proposal being at least as ambiguous as current definition, as others said), please try to be coherent with yourself.
Or may be you didn’t even read your original post? 
Where did that even come from?
It was a mental shortcut but you’re right, it’s not so clear at first glance so I edited the original post to make it more obvious.