Santali (sat-IN) is one of the 22 scheduled languages of india. And yet it does not appear in the language list of Libre office writer.
It is in the list since 2008 already.. If you mean it does not appear selectable as default document language then that is because there is no locale data available. For how to contribute locale data please see https://wiki.documentfoundation.org/LibreOffice_Localization_Guide/How_To_Submit_New_Locale_Data
Set to NEW
*** This bug has been marked as a duplicate of bug 154987 ***