"Walla" is the French Canadian pronunciation of of the
French word "voilà"--a regional pronunciation that trickled down into the States because the two countries share a border.
To chalk it up to a "stupid American" pronunciation because (paraphrased) "
Americans can't pronounce anything not English--and barely that" is a flat out display of
ignorance and a glaring lack of diligent research; it's quite possibly an ironic display of one's own "stupidity".
To borrow from SA's example: "So I was on a cruise around the world and we were heading towards American territory again. This
lady on the ship asked me who the stupidest
people in the world are. Before I could answer, a
stupid guy surfing UrbanDictionary pointed at himself and said 'Walla!' in his best pretend Canadian accent."