My favorite example is that in France, stop signs can say STOP, but in Quebec they say ARRĘT.
They should say ARRĘT in France, too.
EU uniformity.
Seriously?
Not sure, actually. I know differences between signs between European countries were eliminated in the 80s, but I'm neither sure it was the EU that did that and not some voluntary agreement (perhaps larger than EU-wide, the EU was pretty small then) nor do I know what your stop signs said before that.
I know stop signs in China and Spanish America are in the native languages, though.
So... I checked Wikipedia... and they said
"The already-widespread use of the MUTCD [some convention of US states on the issue of road signage] stop sign became law in the United States in 1966. In 1968 this sign was adopted by the Vienna Convention on Road Signs and Signals as part of United Nations Economic Commission for Europe's effort to standardize road travel across borders. The Convention specifies that 'stop' be written in English or the national language and allows an alternative circular yellow sign. Many European countries are party to the Convention. English speaking countries, the exception being India, are not party to the Convention but usually use the red octagonal stop sign per their own standards, like the MUTCD. Even in countries not associated with either standard mentioned above the red octagonal stop sign is often used. Unique types of stop signs may be still be observed in countries like Japan."
So, yeah, nothing to do with the EU.