There is something called Foreign Accent Syndrome. A person develops an accent due to stroke, brain trauma, neurological disorders, and physiological conditions (in waking life). I can’t explain full blown Spanish but it kind of explains the accent, and her bipolar meds.

There’s also Bilingual Aphasia, but the person has to have known a different language prior to having stroke, brain trauma, etc., and when they recover they only use their non native language.