Was your mother a late bilingual? From this paper [1], we know that for late bilinguals, regions for each language are spatially separated in Broca's area. Broca's area is responsible for speaking, while language understanding is attributed to Wernicke's area*. It's possible that the stroke affected only the part of the Broca's region responsible for English speaking. And since language understanding is handled by another region, it wasn't affected.

*For completeness, according to current neurolinguistic models it's more complex than that, more brain regions are actually responsible for speech/understanding.

[1] http://www.nature.com/articles/40623