To change the language of chat widget, go to Manage-->Messages (direct link). Under 'Chat widget language', you can select the language.
On choosing language, all texts associated with the widget will be translated. The preview of widget will be shown in the right pane.
Select 'Save' button at the bottom to reflect the changes.
Note: If you see a wrongly translated text, write to us at email@example.com. If you don't see your language listed, reach us at firstname.lastname@example.org, we will be happy to take your request.