I've read the bit in critiques, and they are different species. It is not even clear they have common ancestry, and almost certain their vocal capacities will be extremely different. Similarly, they are physiologically different enough that straight body language will have a very limited overlap in concepts, and sign? Well, they will presumably all have some manipulatory organs, but dissimilar enough that recognising the symbols with such an "accent" will be slow and cumbersome.
And since language is the framework on which abstract thought is hung, any concept more complex than "don't hurt me" is likely to come out as a very different symbol from the different races.
If they have a common origin; a single intelligent race that evolved, or was evolved, to fit the various ecological niches, there is a hope for an underlying communication logic which they can build on; if, however, they have converged on sapience from different origins, they are unlikely ever to truly understand each other; what would an intelligent cephalopod have to say to us?
Writing develops late in a culture, after the basic symbolic forms are already set; we couldn't expect all the different races to bend their thought forms to one, so expect a band of earnest diplomats to develop a brand new 'pigin', independent of all the original forms.
Assuming writing starts as pictograms (not a safe assumption, by any means) and that the visual abilities of the multiple races are more or less equivalent (neither is that, though Dra'qua seems to use sight rather than sonar for distance viewing), some concepts would be relatively easy – numbers, in particular, and some nouns – while verbs would already be more complicated, as theychange with time, while the picture is static.
Again, assuming the language separates nouns and verbs, which isn't even true for all Earth-human tongues.
Some adjectives would come across; colours, if the two races communicating had reasonably similar vision, comparative sizes, weights and temperatures (easier to explain "bigger" than "big", and just as useful), while others, more abstract, would disappear. I suspect adverbs woud all go, making "I swim fast" "Me water go quicker"; inelegant but comprehensible.
A trading language, surgically removing the inessential, minimising and lowest-common-denominatorising though patterns. Almost certainly, those with vocal languages and similar sound production capabilities would work out time-saving auditory equivalents, and those with similar physiologies signs, and long term trading partners would learn each other's subtilities, but the essential symbol set – a hundred, perhaps two hundred words – would be engraved on stone, static and very, very limited.
And lacking in poetry.