Maybe it's Interlingua. https://en.wikipedia.org/wiki/Interlingua
Most people who speak a latin based language already understand interlingua.
That would be the best chance of getting a majority of the world on the same language. It would include a big part of Europe, all of South and Central America and half of North America
OP probably wasn't aware it was an SDDM issue. Or even what SDDM is, hence the question.