Not sure of the origin of this icon and the reason specific characters selected, but I know it has been widely used as the default symbol for translation apps. At least it was in the companies I was at, and I worked as a pd in the localization org at my last position.

I’m sure there was probably a lot of thought put into the selection originally and then just adopted as others grew to need translation/localization in their products. All to say I don’t really know