Look in export_descr_character_traits.txt and find the GoodDiplomat trait. It's probably going to look something like...
Code:
;------------------------------------------
Trait GoodDiplomat
Characters diplomat
NoGoingBackLevel 6
AntiTraits BadDiplomat
Level Tactful
Description Tactful_desc
EffectsDescription Tactful_effects_desc
Threshold 1
Effect Negotiation 1
Level Courteous
Description Courteous_desc
EffectsDescription Courteous_effects_desc
Threshold 2
Effect Negotiation 2
Level Diplomatic
Description Diplomatic_desc
EffectsDescription Diplomatic_effects_desc
Threshold 4
Effect Negotiation 3
Level Polished_Diplomat
Description Polished_Diplomat_desc
EffectsDescription Polished_Diplomat_effects_desc
Threshold 8
Effect Negotiation 4
Level Expert_Diplomat
Description Expert_Diplomat_desc
EffectsDescription Expert_Diplomat_effects_desc
GainMessage Expert_Diplomat_gain_desc
Threshold 16
Effect Negotiation 5
As you can see, only the Expert_Diplomat level has a GainMessage defined (i.e., the message that drops on the left when that level of trait is acquired). You can add a message for every level, by inserting a GainMessage line with a unique identifier. The text for the identifier goes into text\export_VnV.txt
There are a few other traits that affect Negotiation ability, but you'll have to find those yourself.