I'm afraid that it still may remain a mystery like so many others lost in the past but here is what we DO know according to our 461st Historian whose father was the commanding officer and spoke several languages he says:
It does appear to be Spanish. I checked with Google myself and discovered
something interesting. Starting with "Al Ataque", the detection is that
it's Spanish, but the translation to English only says "Attack". I get
nothing from Italian or Latin. If I start with "To the Attack" and asked
for a translation to Spanish, Italian and Latin, the only one that comes
close is Spanish which gives me "Al Ataque". Looks like it's definitely
Spanish.
As to who came up with it, I still think my father did, but I can't prove
it. I did talk with a veteran, but he can't remember that level of detail.
He was there, but all he remembers is it suddenly appeared. He said he had
come up with the lightning bolt that was on the bomb. The details about who
was involved after that is lost in time.