BreakthroughI can visualize a breakthrough: You work and work, labouring and sweating away, and finally you have made it (with a little help from your friends and maybe even your enemies). You are finally through. Elation! Jubilation! The German language knows how to express this feeling too, it is quite literally the same: Durchbruch.
Only Spanish still resists: Do Spaniards have fewer breakthroughs? That would be a pity, specially if they had fewer or lesser breakthroughs for lack of a word for them (yes, I know: it does not work like that, but anyway). Look what I found as translation for breakthrough: avance, logro, descubrimiento, exitoso. And in case you choose to write break through in two separate words, the result becomes derrumbar or abrirse camino. This is plainly insufficient.