Advanced Details: The words vocabulary and slang

We almost always use the singular forms of these words. So, we will always write vocabulary and slang, without the plural form with -s. The words vocabularies and slangs are very rare.

We will write vocabularies when we are talking about 2 different languages, as in the following sentence: The vocabularies of Spanish and Italian are very similar – they have many words that are almost exactly the same in both languages. Also, for another example, we will write, the slangs of many young people around the world are often very similar, even though these young people are using different native tongue languages.