I'm playing a Magical Child Vigilante. One of my archetype features is a familiar that becomes a variety of advanced familiars as I level. Its base social form is a mouse and its first vigilante form is a Pyrausta.
From my understanding, a familiar gains a language of your choice, unless it knows a language already. As a mouse the familiar understood a language of my choice-- Common-- even though it couldn't speak. As a pyrausta, the familiar can speak and understand the language from its stat block: Draconic. I have no skill points invested in Linguistics.
Does this mean that, when I go into my vigilante identity, my familiar somehow forgets how to understand Common because its only language is now Draconic? Or does it gain Draconic in addition to Common while in that form?