In at least the English translation, the "we" is taken from the translators, ICEL.
Here's what I think -- it was deliberately changed because of that STUPID 'community over all' mindset in the Vat 2 churches. I bet it happened in the last 20 years. I consider it a bastardization of the Creed. Until proven otherwise....
Both the Greek and Latin say "I believe", and both use the accusative case "in one God" which more literally translates as :"into one God" (which is kinda awkward in English".
"Pisteuo en ena Theon"
"Credo in unum Deum"