chat welcome msg ?

Hi, I wanted to know how to translate this message :

Welcome to our chat! You are in the "Lobby" now, but you can pass into any other public room you wish to - take a look at the "All rooms" box. If you have any problems with using this chat, there is a "Help" button on the right at the top (a question icon). Simply click on it and find the answers to your questions.

It's not part of the language file, somebody can help me and tell me where to find and how to translate this message ?