Hi! Where is this string?
Some context here would help 
here the text Start of conversation can we modify it?
Hi!
You can customize all translations with the Custom Translation settings.
Go to Admin > General > Translations, and add them as a Json object per languague:
{ "en": { "Start_of_conversation": "Custom New Conversation Message" }, "pt": { "Start_of_conversation": "Nova mensagem personalizada" }}
In order to discover which key to translate (for example, Start_of_conversation), you can search for the sentence in our codebase at Github, like so:
Hey, how to find all objects and have them in json file ? to check where I have translation and where i don’t have, like in .po file ?