Can Dragonbait read or write Common, since he is able to understand it?

Question

In D&D 5e, Tomb of Annihilation includes Dragonbait, a saurial. His stat block claims that he can understand Common but not speak it, due to the strange way in which saurials communicate. From Tomb of Annihilation, p. 218:

Languages understands Common but can't speak

Is there any evidence, either in 5e (which I assume is just what's presented in Tomb of Annihilation) or anything from previous editions of D&D, that suggests that Dragonbait (whether it's about Dragonbait specifically or saurials generally) can read or write Common, since he can apparently understand it?

Answer 1

While not clearly stated in ToA, it is unlikely--especially looking at other lore sources such as the Finder's Stone Trilogy, where Dragonbait hails from.

How he communicates...

The saurial’s language is a combination of clicks and whistles beyond the human range of hearing, so Dragonbait tends to rely on scents to communicate emotions. To be understood by most speaking creatures, he must “shout” a scent. Dragonbait’s known scents include: brimstone (confusion), roses (sadness), lemon (pleasure or joy), baked bread (anger), violets (danger or fear), honeysuckle (tenderness or concern), wood smoke (devotion or piety), tar (victory or celebration), and ham (nervousness or worry).

Source

Novel sources...

If you then continue on to look at his history in the novels from which he originates (the Finder's Stone trilogy)...he regularly has trouble communicating with those around him. He can understand them, but has significant difficulty responding.

The language barrier was a severe enough issue that Alias took it upon herself to teach Dragonbait Thieves' Hand Cant...basically a very rudimentary form of sign language that is, according to that novel, part of Thieves' Cant. Eventually, because Hand Cant was fairly rudimentary...Alias ended up casting a Permanent Tongues Spell on herself so that she could understand Dragonbait and communicate with him in his own language.

Were he able to read and write Common, it is very unlikely that he would have struggled so much to communicate--and that so much effort would have been put into working around the language barrier. He'd just use his claws to scratch words into the dirt, or carry scraps of parchment and some charcoal.

Answer 2

There is evidence that saurials can read the languages they know

In Dragon Magazine #292 (thanks to this answer for pointing me in this direction), it describes the lore and mechanics of saurials. Specifically, under each subrace, each has an Automatic Languages section, which reads (p. 41, taken from Finheads, since that's the kind of saurial that Dragonbait is, but it says the same thing for each subrace):

Automatic Languages: Draconic.
Bonus Languages: Common, Elven, Sylvan, and Celestial. Saurials have difficulty with other languages, and although they can understand and read all the bonus languages they know, they cannot speak them without spending skill points.

Since all saurials can apparently read the langauges they know, this would imply that Dragonbait would likely be able to at least read any languages he knows, even if he can't reply.

However, it also mentions that saurials can speak if they spend "skill points", which goes against the point of saurial's language somewhat, and even points out in a section specifically about Dragonbait (p. 43) that, although he initially lacked the ability to speak the languages of another race, he:

... learned to speak the Common tongue ...

which contradicts other sources about saurials generally (such as the information found in 2e from this answer).