Why are you stuck? How many results do you get for “narrative_bot.new”?
Finding texts is easiest by searching for the text you want to change. So, to change the first message, you can search for “I am only a robot” (or the French version of that). This is also explained in Customizing Discobot for your site.
Then you could search for the next one. Completing the tutorial and then searching for the texts from it can be helpful because then you know the order of the messages.
You can also find similar texts based on the key. For example, you take discourse_narrative_bot.new_user_narrative.hello.message and remove a part of it to search for that. If you remove “message” from the key, you will also find the title. If you also remove “hello,”, you will find more messages than the “hello” message.
You can also find all texts on GitHub. Most plugins have a client and a server file. The messages are in the server file, but I find it quite difficult to get keys from those files. I prefer Crowdin for searching through more texts than the Discourse interface returns, but usually searching and using the keys is enough.