One can use http://extensions.services.openoffice.org/en/project/Russian_dictionary_AOT_base_ieyo dictionary to add Russian spellcheck option to Spark. But currently Spark is throwing exceptions when trying to use this dictionary (see the linked ticket).
One has to extract the oxt file and rename aff and dic files to ru_RU. Then remove everything else except for aff, dic, copyright and lgpl files and zip it into ru_RU.zip. Then it can be added to src\plugins\spelling\src\resources\dictionary directory.