To be fair, this is common of many bilinguals when the other language is not in common use in their lives. I personally know at least 8 bilinguals ranging from

To be fair, this is common of many bilinguals when the other language is not in common use in their lives. I personally know at least 8 bilinguals ranging from Chinese to Zulu that can understand the language, once-upon-a-time spoke it well, are spoken to in the language, but respond in English. It happens frequently when the only time they ever use the language is with their parents/family and their parents can understand English just fine.