To wish someone a Happy Valentine’s Day in Ukrainian language you can simply say “З Днем Валентина!” or “З Днем Святого Валентина!”. Both options are correct. The last one is translated as “Happy St.Valentine’s Day”.
Happy Valentine’s Day in Ukrainian: images and gifs
Happy St. Valentine’s Day in Ukrainian: wishes
Another common way of greeting on this day us to say “З Днем Закоханих” which would be literally translated as mean “Happy Day of people who are in love”. Maybe it doesn’t sound that good in English translation, but it is a very common phrase in Ukraine.
Щасливого Дня Святого Валентина