Astra knows ZERO Octarian (she didn't even realize octolings were different than inklings till she was like 20 lmao) - Saph and Cerise both learned and became fluent in Inkling after they reached the surface when they were teens :> Nowadays they all typically talk in Inkling on a regular basis, except when Saph and Cerise talk about more serious topics that they don't want Astra to overhear (they've never fully talked about their time in the metro with her)

The language barrier was a problem when they all first met, Saph and Cerise could only understand little snippets of Astra's ramblings as she excitedly showed them different parts of Inkopolis, but they could generally figure out what she meant. Once they got access to the internet tho, Cerise got the hang of it very quickly and then helped Saph figure it out too. They were probably pretty close to fluent within a year or so :3