Imagine aliens coming to Earth and having translators that work perfectly. Except they don’t pick up on tone.
Tone has a HUGE impact on a message. Consider the following sentence:
“You look nice today.”
Now repeat it stressing each word one by one.
“YOU look nice today”, implying someone else that you are probably indicating with your body or to whom you’d previously referred does not.
“You LOOK nice today”, implying that you don’t smell/sound it.
“You look NICE today”, thus turning what would otherwise have been a casual remark into a compliment. You don’t just look nice. You look damn fine.
“You look nice, TODAY”, which is clearly an insult purpoiting that you usually look like crap. Damning by faint praise, as they say.
And all of these are possible - and wildly differing - meanings to a simple four word sentence.
In this scenario, super secret plans could be discussed in front of the aliens with them being none the wiser simply by saying it à la Mean Girls. Should war between the two factions emerge, humans would win by the power of passive-aggressive bitching