Well, you could think of it like this:
The information that is transmitted by the Smartgun system (in the gun) is normally interpreted by dedicated software/hardware that is run by the Smartlink vision enhancement. The reason why it is required to be in the vision enhancement and can't be run by just a commlink is because of dedicated firmware in the vision enhancement that does things like tracking head movement to line up your sights etc.
For a Drone, the information that is being transmitted by the smartgun system is just data that the drone doesn't know what to do with if it isn't running the Smartsoft program. That program lets the drone understand the data and allows it to function as if it has a Smartlink. Without it, the drone's dogbrain just doesn't know how to utilize the extra data.
The way I see it, for Riggers, you have two options. 1) The Smartsoft system (or something equivalent) is still required. The program helps translate the data from the smartgun system and aids the Rigger as normal. 2) Because it is running through the computer system and being translated through the Control Rig for the Rigger, no extra software is required. The information is translated appropriately for a Rigger that is jumped in and allows them to always function with the effects of a Smartlink as long as the gun has the Smartgun system.
Not sure which way is meant to be the intended rulings, but those seem like the most likely options to me.
And as far as Linguasofts go, I would say, yeah a commlink can probably do some translations for you, but a Linguasoft specifically allows the user with a Skilljack to act as if they actually know the language. Running translation through a commlink is going to be much more clunky.