Are slang words allowed in Scrabble?
You can check for yourself at the Official Scrabble Dictionary.  According to that site:


za is a legal word
lez is not (Keep reading)


Words With Friends uses the Enhanced North American Benchmark Lexicon with a few additions per their website.  ENABLE is a public domain word list for use with word programs.

Thanks to @thesun's research it is obvious that the website is not the final word for Scrabble tournaments.  Apparently the Scrabble folks censor their website, and books of around 200 offensive words.

To get the complete list you have to join the National Scrabble Association and pay for it.  For most tournaments that will be the list used.  If the tournament is televised then perhaps those 200 offensive words will not be allowed.