In general, yeah, I agree that it's like cheating to tell the audience what the POV can't understand. I like the way missmollyetc has Geno not understanding English in one of her fics:

"Everybody knows!" Sid yelled, stepping up and into Geno's space. He grabbed Geno's shirt and shook him. "Everybody fucking--fuck, how long before...talk and...press...fucking...Deadspin or—"

"Too fast," Geno said, smacking at Sid's arm. "You go too fast again."

It occurs to me that also if the characters don't even notice how they're switching between languages, it's going to have to be someone else that notices - a monolingual POV character, or just someone telling them to stick to one language at a time. Something like that. Make the reader realize at the same time the character does, you know?