The figurative language in the above sentence is Personification.
Figurative language is the language that uses terms, words, or expressions that has a different meaning from the literal meaning. These are the types of figurative language; Metaphor, Simile, Personification, Hyperbole, Symbolism, Alliteration, and Onomatopoeia.
The term "mental slavery" signifies the use of Personification as the figurative language.
Personification is giving human characteristics to inanimate objects, animals, or ideas.
In the above statement, slavery is associated with humans was given as a characteristic to an idea "mental".