Have you seen such a string before, lowercase en uppercase
US? It stands for English‑United States.
But what exactly does that mean?
The first part, the en, is the language designator.
En stands for English, as I already mentioned, but there are about 183
more options as defined in the ISO 639 standard. Depending on the
region, en might not be specific enough since there are notable
differences between say American English and British English; therefore,
a region designator comes in handy.
US stands for United States. But once again there are many
more, 248 more to be exact, this time defined in the ISO
3166 standard. For Great Britain,
the region designator is GB, which would lead to en‑GB in
this case. The end result contains both the language and
the region and is called locale.
Some platforms also include a charset into their locales.
For instance,
there are locales for simplified Chinese and for traditional Chinese. In some
countries, there are locales for Cyrillic characters and Latin characters,
such as Uzbekistan. For our purposes, however,
the language and the region are good enough. Depending on your site,
sometimes only supporting one language,
regardless of the region, may suffice. People from Great Britain should
understand American English very well, as do folks from Austria when they read
German. Once regional specialties such as units or currencies come in, you can't really do without region designators.