technically all words in katakana are slang, foreign, or just plain wierd. yor comp is like a virtual dictionary; sad to say, there is still no way of updating written slang for the japanese word document. it would be too much for one simple system like word. same when writing slang in english! u either get spell corrections, or if u spelled it right, u get a grammar correction telling u that u used it wrong in the sentence.
all i do is use the hyphen button when using katakana!
hope that helps!