The Octarian language is a fictional language spoken primarily by Octolings and Octarians. In Tentakeel Outpost, Marie will sometimes say, "I'd tell you what's written on this sign, but my Octarian is pretty rusty." which indicates that Octarian is the official Octoling language.
The Octarian language like the Inkling language appears to use two scripts which have been translated into English by fans. They appear to follow the English writing format when translated.