I think that expression is more literary or poetic and is not commonly used (at least, not as common as say a "gaggle of geese"). There's also a book and movie with that title. I found a little bit on the
origin of the phrase here for anyone who's interested.
Back to DB: the easiest translation has to be "dragon" in English to "dragón" in Spanish. It doesn't get much easier than that. :) Though they don't try to translate the character names much, even though they just about all have meanings in English. I guess they do that to avoid having to re-label toys for the different markets even if it means the Easter Eggs in the names get lost.
Though I did notice on the Happy Meal toys that they changed two of the dragon names in French (the bags the toys come in are labeled in English, Spanish and French). Decepshun becomes "Decepta" and Propheci becomes "Prophetie". All the rest of the names they left the same in all three languages. So they stuck with Hyve, Shock-ra and Ferno, for example, even in French. Interesting. :)