Je déteste le français. To say "I hate French" (as in the French language) in French, you would say, "Je déteste le français." If you want to say "I hate the French" (as in the French people), you would say instead, "Je déteste les français."
If one is wanting to purchase congrats postcards these can often be found in card shops such as Hallmark. There are also a number of sites that offer free congrats postcards in online format such as 123 Greetings, and Bluemountain. One can also download printable cards from Greetings Island.