Not by the EU, I think it goes back to the fifties or sixties when a UN related conference tried to get a lot of countries around the world to use similar traffic signs. I imagine not so much the letters saying "stop" but the shape and colors of the signs.
Yes, just looked it up. Viena in 1969, on a UN conference.
Don't know why they are in english in Spain and Portugal, though, maybe because cars arrived in Portugal later than in England or France and it maybe made sense to use the same signs?
https://en.wikipedia.org/wiki/Vienna_Convention_on_Road_Signs_and_Signals#Road_signs