Albino Bruxa Tongue is, or rather was, an ingredient used to make the Decoction of the Grasses, the potion used in the first part of the Trial of the Grasses by the School of the Wolf to create witchers.
A table in the Kaer Morhen keep provides this, along with two other rare ingredients (Forktail spinal fluid and Manticore poison gland) so Yennefer can perform the first part of the Trial of the Grasses, making Uma receptive to magic and hopefully lift the curse.
Associated quest 
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