How do I improve my code (game of dialogues and choices)

If you really want to scale the amount of dialogue and choices, you need a system that reads the dialogue/choices from external files and puts them in Construct in a way that's easy to reference and read (e.g. storing them in Dictionaries). Also, you need to think through how a dialogue is intialised, and how a choice leads to another dialogue. It involves some care in planning and executing, but it will serve as the framework for any kind of dialogue, and you'll be able to edit/revise your text and the flow of the dialogue more easily than if you were to do everything within events.

(FYI, I am making an ARPG prototype -- in C2 -- that has a lot of dialogue. I've implemented my own dialogue system which is partly based on a nodal graph format. But it was originally just a specially formatted technical writer's nomenclature, and this is where I'm basing my advice.)

I think you should think about designing a text-based writing format that makes sense to you. I'll give a simple example:

init_topic=intro

intro.text=You are in a hallway. Choose a door.
intro.choices=door_a,door_b
door_a.choicetext=Door A
door_b.choicetext=Door B

door_a.text=You enter a room with a lever on the floor. What do you do?
door_a.choices=lever,nothing

lever.choicetext=Triggers the lever
nothing.choicetext=Does nothing and returns

lever.text=A trapdoor opens into the ceiling and a zombie falls right in front of him.
lever.choices=fight

door_a.text=You enter a room with a lever on the floor. What do you do?
door_a.choices=lever,nothing

door_b.text=You enter an empty chamber, everything in this place is a picture on the wall.
door_b.choices=examine_picture,come_back

examine_picture.text=Nothing happens
examine_picture.choices=come_back

come_back.text=You enter a room with a lever on the floor. What do you do?
come_back.choices=door_a,door_b

dialogue_dict['init_topic'] = 'intro'

topic = dialogue_dict.Get('init_topic')
topic_text = topic&'.text'
choice_key = topic&'.choices'
for(i=0;i<tokencount(choice_key,',');i++)
{
 choice_text = tokenat(choice_key, i, ',')
 DisplayThisChoice(choice_text[/ic])
}