Would it be nice to have an international whole-world standard for traffic sign tagging? Yes! It would be very nice indeed!

Unfortunately, such an international standard is very unlikely to ever happen.
yopaseopor has tried many international traffic sign proposals, which got rejected or never came to fruition for many reasons.

A month or so ago, yopaseopor released for comments a proposal for using "traffic_sign:id" for the national traffic sign codes (https://community.openstreetmap.org/t/new-proposal-about-traffic-sign-international-code-mapping/125438). The results were mixed, some liked it, but others opposed it because it will conflict with their local systems; and it caused a big and long debate on the forum.

If something as simple as moving a single tag, caused this much of a debate, then discussing an entire international traffic sign tagging system is a literally impossible mission that will lead to absolutely nowhere.

These are the reasons why I think that international standards in the world of traffic signs are futile. Taking into consideration all the signs, organisation systems of these signs, exceptions to those systems, icons, and placement of everything is a colossal task.
And this is why I think that regional/national traffic sign tagging schemes are, for now, the way to go, because the people from said region/territory that know about it can steer it into the right direction, and it will not be necessary to consider all the weird edge-cases of the entire world.

And yes, you are absolutely correct that many countries, not only in Europe, but in the entire world, have very similar traffic signs, thanks to the Vienna Convention. But this doesn't mean that everybody who follows the convention has identical signs, every country has a different combo of symbols, fonts, text, icons, colours and even shapes. And also, the Vienna Convention goes out the window in the destination sign situation, where every country is a whole different world, so the convention can do very little with roundabouts and intersection signs.

An international debate and/or international proposal for how we should map Catalan traffic signs only in Catalonia is nonsense. Catalonia-specific things should be proposed, debated and approved by users in Catalonia only; it doesn't make sense to involve other mappers from the other side of the world that have no idea of how things work in Catalonia.

If there were two or more competing local standards, with no way to set things clear of which to use, an internation debate could be a tiebreaker to solve the situation; but that's not the case here. Catalonia had no local traffic sign standard before, we are creating one. As easy as that.

Other countries and territories in the world do also have their own ways of tagging and mapping traffic signs, like Finland, for example (https://wiki.openstreetmap.org/wiki/Finland/Traffic_signs#Traffic_sign_mapping_in_Finland). Do you also chase the commits of every new Finland traffic sign that gets added to OSM?

Regarding the fact that only users are working on this system, I get your point. But you may also know that not so many people are interested in traffic signs, so there's nothing that we can do about this. I'm not a fan of taking people hostage and forcing them to read the several manuals of Spanish and Catalan traffic sign specifications, so I'm afraid that this situation may not change in the near future... xD At least, two people actively working for a standard is better than one.

And in every debate and conversation that we've had on the Catalan community Telegram group chat so far, no one has opposed this standard so far.
And honestly, the only one here that seems to have objections and complaints about this standard is you. And (no offence) but you are not part of the Catalan community, so the number of opposing people goes back down to 0.

In OSM there are many things that are regional and national-specific.
For example: in values above 2 of admin_level= tag, the organisation varies greatly country per country. Check this massive table about how admin_level works in different countries: https://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative#Table_:_Admin_level_for_all_countries 

It makes no sense to involve the international community in something that is Catalonia-specific. But still, if international feedback comes our way, we'll consider it and implement it if we find it useful.

It's true that debates in changeset comments are hard, but again, here's the forum thread where we can discuss this better:
https://community.openstreetmap.org/t/proposta-senyals-transit-catalunya/127587

And here is the WIP wiki page of this standard (everything that I told you about the tagging system is also here):
https://wiki.openstreetmap.org/wiki/User:Yopaseopor/Proposta_senyals_tr%C3%A0nsit_Catalunya