When a visitor on your website clicks on the chat icon, they are greeted with a welcome message. If you're offline at that moment and you've enabled offline display, then the visitors also see an offline message letting them know you're currently unavailable and asking for their email so you can contact them later. These messages are shown to your visitors in their browser's default language if the language is English, French, Portuguese, Spanish, German, or Italian. (For all other languages they will see the messages in English).
If you want to change these default messages, then you need to add the following lines to your Sendinblue tracker, as shown in the image below:
window.sib.custom_welcome_message = 'Your custom welcome message string';
window.sib.custom_offline_message = 'Your custom offline message string';
You may need to ask your web developer to do this configuration, which should only take a few minutes.
You can locate the chat script from the Chat by clicking on Settings > Messaging channels > Website Chatbot.
Please note that you can also choose not to display any or both of these messages by simply leaving the value for these variables as blank, e.g:
window.sib.custom_welcome_message = '';