I don't know how a stroke would affect speaking a tonal language but I can tell you that my Cantonese speaking patients with language impairments had much more difficulty understanding my "Cantonese" than those with truly intact language. (my Cantonese is limited to: good morning, how are you, slow down, stand up, sit down, does it hurt, and are you dizzy. functional but not very sociable.)
On a slight tangent I would frequently have patients who spoke English to some degree before their stroke but not after yet their first language was left unimpaired. Even one lovely lady who had not spoken French since leaving France at age 5. After her stroke she became fluent at French but not a word of English. She started English lessons and her husband of 54 years started French lessons.
