A bilingual conversation can change direction because one language offers a gentler doorway.

Some things are easier to say in one language than another. A phrase that feels too naked in your first tongue can pass more easily in your second, or the reverse: the childhood language carries the tenderness that the adult one cannot manage. Switching partway through is often not showing off or laziness; it is somebody finding the door that opens.

If you share more than one language with a person, let the switch happen without commenting on it. Notice which subjects pull you into which language. That pattern usually marks where the difficult material is kept.