My wife's first language is different from mine. Her family always switches languages around me and then apologizes. For two years, I've been secretly learning it through apps, videos, and help from a coworker.
I understand much more than they think. Thankfully, I haven't discovered any terrible secrets. Mostly they discuss food and relatives. I'm waiting until our anniversary to speak to her properly in her language. The hardest part is pretending not to understand when she complains about me to her sister while I'm sitting nearby.