The Program pane offers a separate tab for each of the supported scripting languages (JavaScript, VBScript, AppleScript). Each tab contains a multi-line text editor for entering a script program in the corresponding language. The text editors use syntax coloring appropriate for the language being entered. The JavaScript editor also supports function name & argument completion as shown above.
You can select the scripting languages being included in the script package (and thus enable/disable the corresponding tabs) by setting appropriate values for the "include xxxScript" properties in the Declaration pane. Also see Writing a script.