Funnily enough, I had to do this exact same thing for a project I'm working on not an hour later, and I think I've come up with a more elegant workaround.
In the Items field, use:
If(ENG, AddColumns(YourList, "ENGFR", ENG_Column), AddColumns(YourList, "ENGFR", FR_Column))
Then, set the Search Field to be ENGFR, and if your searching based on ENG, ENGFR will be the ENG_Column, and if not it will be the FR_Column