Hello there, and sorry for the late reply!
The mission dialogue can be found in dialog/conversations.
It usually follows pattern below

c_topic_index_typeofconversation_character_lxmy

typeofconversation is usually "noboxconv" or "convinit", can also be "tutorial"
character is usually the dialogue code of your character
LXMY is your level and mission (i.e l1m1)
I hope this gives you a better sight at the things and helps you find the lines you're looking for, easier!