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How to change the texts

Go to the Messages parameters page and look for Go!Chat SDK section. For each language you can override the texts of the session. The complete list of languages is:
Code Language
ar-AEArabic
cz-CZCzech
da-DKDanish
de-DEGerman
en-USEnglish
es-ESSpanish
fi-FIFinnish
fr-FRFrench
hu-HUHungarian
id-IDIndonesian
it-ITItalian
nl-NLDutch
ro-RORomanian
ru-RURussian
sv-SESwedish
tr-TRTurkish

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About categories (tags)

You can set the label of each tag. If you have this tags codes: ["supportaccount", "supporttransactions"], you can set the labels like this: 
{
  "en-US": {
    "messages": {
      "tags": {
        "tag_supportaccount": "Account issues",
        "tag_supporttransactions": "Transactions issues"
      }
    }
  },
  "fr-FR": {
    "messages": {
      "tags": {
        "tag_supportaccount": "Compte issues",
        "tag_supporttransactions": "Transactions issues"
      }
    }
  }
}
Here is an exhaustive list of sentences you can override, use it as an example: 
{
  "en-US": {
    "common": {
      "powered_by": "Powered by ",
      "account_error": "We can't load your discussion, try reloading the page.",
      "back": "Go back",
      "confirm": {
        "title": "Confirm action ?",
        "subtitle": "Confirm action by clicking Confirm.",
        "cancel": "Cancel",
        "confirm": "Confirm"
      }
    },
    "threads": {
      "new_thread": "New discussion",
      "open": "Open",
      "waiting": "Action needed",
      "resolved": "Resolved"
    },
    "messages": {
      "tags": {
        "prefix": "About: ",
        "select": "Select a category",
        "tag_": "Other"
      },
      "select": "Select a discussion from the list on the left.",
      "created_at": "Discussion started ",
      "updated_at": "Last updated ",
      "to": "To",
      "cc": "Cc",
      "received": "Received",
      "sent": "Sent",
      "send": "Send",
      "attachment": "Documents",
      "subject_placeholder": "Subject",
      "placeholder": "Message...",
      "title": "Your message",
      "error": "An error occurred while sending your message, please try reloading the page.",
      "attachment_size_error": "The file is too large (maximum size: 10Mo)"
    }
  }
}