[dahdum]

I've always taken that as the characters not knowing which language is being spoken. If it's been established I expect to see [SPEAKING MANDARIN] or some such.

They don't subtitle it because the audience isn't expected to know what's being said.

[CRConrad]

But when it's subtitled in the movie itself (which is then covered by the TV's closed-captioning system), the audience is clearly expected to get to know what's being said.