Hi again. I've downloaded three files from https://github.com/LibreOffice/dictionaries/tree/master/ru_RU : hyph_ru_RU.dic , ru_RU.aff and ru_RU.dic ( as Walt.Farrell advised in other forum thread).
I'm on Win 10. So .. then I've copied them into Program Data > Affinity > Common > 1.0 > Dictionaries > ru_RU folder. I created "ru_RU" folder cause otherwise AD doesn't t see russian language at all.