I like giving bad guys a "catch phrase" in my campaign notes, always good stuff.
Slaadi don't say anything as they're communicating in Telepathy, which means they won't sound like an idiot referring to themselves in the 3rd person. Rather, the characters will be hit with pure thoughts. This could be images, sounds, smells, pretty much anything that sums it up. Prior edition descriptions of Telepathy have said it's "communication" (so players can communicate back), but just like communication, it's tough to carry on 5 different conversations.
For example, rather than say "I'm going to rip you apart," the slaad instead bombards the character with imagery of the character being ripped apart by the slaad's oversized and exaggerated claws, blood spraying everywhere, with the iron-tang smell in the air. The thought is conveyed in a way the target would understand. If the target is a Modron, it instead would perceive its parts being disassembled and placed out of order by the big claws, no blood.
But, it's unlikely a slaad would communicate a foe being ripped apart, and that's because of their ecology.
To know what the typical slaadi might say, you can look why they exist (going back to my Planescape days). The low INT ones were red & blue, the grunt soldiers tasked with scavenging battlefields of the Blood War for food and wounded prisoners to impregnate with eggs or curse. They'd like to tear down all the established hierarchies to make themselves more powerful, but they don't organize well and don't agree on who should follow who other than who tends to be more brutishly powerful. They also decorate their foreheads with tattoos of status and accomplishment, so Red and Blue won't have many at all. Both exist to procreate by infesting victims, then taking them to prison camps to make sure they give "birth." It's the only way they can reproduce, and it's an overriding aspect of their existence. So, slaadi aren't going to be thinking about ripping you apart. They're thinking how good a host you're going to make.
Red Slaad: Lowest of the low, they like to gang up in numbers and torment/slaughter smaller groups. They're predictable and uninspired and probably not ever up for simple conversation. But, slaadi usually want prisoners as you can't birth more of them (power in numbers) from dead people. So, a desperate hunger image, a need to see claws rip into skin, to open a wound, to see blood flow, to hear that glorious sound of a wounded creature on a battlefield that signifies weakness.
Blue Slaad: If a slaad took troops to war, it'd take the Blue. Like reds, they are vicious bullies. Perhaps go with their nightmarish prison camps. Impending excitement to see the characters writhing on the floor of a urine and feces smeared cage, to hear the laments of these weak, fleshy monkeys, and to know they've served their purpose, combined with the above excitement at a wound.
All in all, they're not really going to be up for scintillating conversation, nor clever taunts. To them, the players represent one thing: a host.