My wife still does it. There are words in both languages that there is no substitute for, so they just go back and forth. But the craziest thing is when she wakes up in the middle of the night after a bad dream speaking Hmong to me... Trips me out. Only happened a couple times though.
My parents spoke Yiddish as first language but never taught us, so they could talk about things they didn't want us to know about.
I took German for all four years in high school and one term of upper division German in college over half a century ago and still remember quite a bit.
I took spanish… in the construction field. Most words i learned i cant repeat. Ut man we had some laughs on the job site “Damn Gringo is speaking spanish again” “…ahh gringo pay lamay la culo, pincha dumadra” (im sure thats grossly missspelled)