Write the letter in Ukrainian, although English would be okay, as they will find someone who will translate it for them. The envelope must be addressed in Ukrainian, with also 'Ukraine' in English for Canada Post. Seeing that there are so many Tashchak's in your village, make sure that you ask them questions that you think they should know. Wait for them to tell you about the family, not you telling them. They may even have some family pictures sent to them from Canada. Be cautious, you want to connect with the right family.
Good luck!