Pycckuu_.srd is the correct file for 3790. RU_ru.srd is for 2012 devices [3490 etc], if you also have it in the ASR folder you should remove it. WebUpdater or GarminExpress will offer you the latest Pycckuu_.srd if you have an old version on board. If you have any previous ASR/SRX files on-board for older map versions also remove them leaving only the current map's 04B42GHT.ASR & 04B42GHS.SRX.

You also need the correct Russian text file and TTS voice for your 3790 loaded and selected. If still not working, hard reset to reselect locale [backup favourites first].