The language popup, at the left above the text area in the script window, determines the OSA (Open Scripting Architecture) language that the script will use.

Mac OS X provides one OSA language — AppleScript. Script Debugger supplies another OSA language:

• AppleScript Debugger X. This is the special version of AppleScript that implements Script Debugger’s debugging features. Whenever debugging mode is turned on, this popup will be set to AppleScript Debugger X.

Additionally, Late Night Software distributes the JavaScript OSA component, which makes JavaScript an OSA language (and gives it the ability to send and receive Apple events).