Now we have two standards:

1. 2-letter codes with ISO-639-1 e.g. en
2. 3-letter codes with ISO 639-2 e.g. eng.

HTML meta headers such as the following example use either a two letter code such as en or a double barrelled code such as en- gb to specify which dialect, in this case the country code gb for Great Britain — the Queen’s English.

<!-- meta tags to describe the language of the HTML document -->

<!-- English -->
<meta http-equiv="Content-Language" content="en">

<!-- the Queen's English -->
<meta http-equiv="Content-Language" content="en-gb">

<!-- Esperanto -->
<meta http-equiv="Content-Language" content="eo">

Sometimes you will even see a triple to describe a locale language: e.g. sr_ YU_CYR with a variant after the country.