I often do that with Spanish, quietly listen then partake of the conversation at the end! Throws em off for sure! Although honestly in this part of the world there is a lot of folks that are bilingual Spanish and English.
My mil was a french teacher for her whole career... Retired as head of language department at Marygrove College in Detroit...
While standing in line at the bank once, the guys behind her were speaking french and talkin' **** about the others in line... She kept quiet and acted like she didn't know what they were saying... When she was next in line to go to the window, one of them made a grammatical error... She corrected his grammer in french and then walked to the teller window....