The language key used to fetch a string. If no string is found for the key, then the key is returned as the string.

An array of variables to use for substitution in your string. For example, if your string is "This %s belongs to %s" and you pass ['Pizza', 'Bob'] for vars, the string returned will be "This Pizza belongs to Bob".

The integer to use for pluralization. This is used if you provide different forms of your string based on the pluralization. For example, your lang key might define the following ['I have one index', 'I have many indices']. If you pass a value of 1 for vars then the first string will be returned by the translate function, if you pass a value of more than 1, then the second form will be returned.