Millie is deaf in one ear. Born with partial hearing loss, which then turned into full deafness following years of tubes, she doesn't always hear the director's "action" cue when shooting a scene unless it's loud.
Millie Bobby Brown was born in Spain so, it's natural to speak in Spanish. But Millie was born to a British family. Her father, Robert Brown, and her mother, Kelly Brown, both are British. So, she does not know Spanish.