InDesign files can be translated in Smartcat but before the files can be processed for translation they need to be converted from a binary format to different formats. These formats are also InDesign formats and the conversion can be done from within InDesign itself. Depending on the InDesign version, files will need to be converted to the .inx format or the .idml format. Both these formats can be processed using Smartcat.
There is more information about InDesign formats and save options here.