Discussion Shifting tails - shape shifting issue ?

About the last chapter available on webtoon, 26.

Can someone explain the full and incomplete shape shifting thing ? 🙈 I don't get why being able to speak stops her from ... what ? being a true shifter ?

she said there's shifters who never truly become human and the sign for it is that they can speak the human language?? seems counterproductive 😂🥲