I don't know where to start but would like to know what languages the extensions can be written in. I'd also like to know if there are fixed pre-programmed user exits or if those can be added by the programmer?
Within SAP itself, extensions will be written in ABAP. However, SAP can call and be called by external applications, and these can be written in a very wide variety of languages, like C, Java, C++, .Net etc.
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