Because Yiddish is so similar to German in grammar and vocabulary, it's quite easy to misunderstand Yiddish terms. The meaning of the word can vary widely based on some Jewish joke, sometimes with roots in mysticism or ancient Hebrew vocabulary. Often, words were chosen to be intended as an insult to the christian governments that gave Jews second class citizen status, as well. So sure, you can look up Christmas in your handy copy of Weinreich, (who doesn't have one?) but you won't really understand the translation at all. It's this kind of double entendre that makes the translation completely worthless.
The thought of non-Jews going around wishing each other a 'freyleche nitl' has me cracking up now. :D