You only have use the FoS Command Trait if you take a Saurus as your General. K.O have several subfactions that function the same way.
It says it directly about listing the command trait: "A Fangs of Sotek Saurus General must have this command trait instead of the one listed on page 57" -Seraphon Battletometome 2020 pg64. Top Right column of Fangs of Sotek Abilities. Which means only a Saurus general must have the FoS command trait. However, it doesn't say that a Saurus must be your general. So, if you pick a Slann or Skink then you still pick your general's command trait from their respective list.
I think what hes asking is that a Saurus must have that trait fair enough, but what's stopping other non-Saurus generals from taking it?
Nothing in RAW. Based on the flavor text and the bold restriction test it is pretty easy to see the langue is identical to the command traits from the Saurus section. I think a strong argument can be made that a Skink or Slann is able to choose the command trait if they wanted. Not sure all TO's would buy that, but enough of them would.