The Problems With Using Real-World Languages in Movies

Gibberish is the worst option outside of a parody. If done poorly enough, even casual listening will tell you that these are English speaking actors making it up on the spot. Some voice modulation can be fun as a gimmick, but without meaningful words behind it it remains just a gimmick. It may be useful for comedy or where you don't really want to have a fully realized culture and language,. But it's definitely not a replacement for a conlang.