I was always under the assumption that the actors in the majority of the spags delivered their lines in their native language and the the whole thing would then be dubbed into Italian first, and then English, Deutsch and whatever else later. Subsequently, there was no “original language” as such, although of course the movies would’ve been written in Italian to begin with. I don’t think I learned that anywhere, I just kind-of assumed that’s how they did it.
We’ve had the subs-or-dubs debate many times already I’m sure but for what it’s worth: I prefer my spags with an English dub and will only watch them without one if one is not available (e.g. Death Sentence), since they’re mostly set in an English-language environment. The characters wouldn’t/shouldn’t be speaking in another language (other than Spanish for any scenes of Mexican interaction). For the same reasons, I prefer my animes in Japanese with English subs, although I’m slightly more tolerant of an English dub on a major title since they’ll probably have pushed the boat out and made a proper high-quality track (e.g. Cowboy Bebop) which, in itself, may be well worth checking out (I’d still ultimately prefer the Japanese track though). Similarly, it bugs me that, say, Cross of Iron is in English with a few half-hearted clichéd German “accents” thrown in, to remind us that these fellows are Germans. Have them talk in German, then! I’d imagine that’s what German soldiers might have done!