"Have a nice weekend."
It should never be accepted. It's a novelty, a borrowing from English, used only in the last decade by a few people, and it sounds stupid to most native Czechs. It works perfectly fine without the "měj" (have). If you want to add anything, add "přeji" (I wish) for a more formal or nice-sounding version.
I second VladaFu's suggestion. The traditional Czech keyboard has a QWERTZ layout (like the German keyboard). I used to have trouble with Y and Z everytime I switched from Czech to English and back. But ever since I started using the QWERTY layout in Czech (it's the same full Czech keyboard with all the diacritics, only Y and Z are switched), I'm much more at ease when switching keyboards. ;)
"Hezký konec týdne" is fine, it sounds like "el fin de semana". But note that "víkend" is a really common word in Czech, it's just what people say.
I know some people who have been using "volník" or "volníček" instead for the last 20 or so years, but it really hasn't caught on and a lot of people wouldn't know what that's supposed to mean, although it's quite easy to guess, given the right context.