Clarify APP_LOCALE

[niccokunzmann]

APP_LOCALE can be set to en but it can also have other values.
Which are the values that can be used here?

en-US
en-GB
de-DE
de

Which format do the locales take?

To close this:

• edit the docs with a link to all possible locales
• why would I set this? Is the UI only in one language

See also:

• https://docs.pixelfed.org/running-pixelfed/configuration.html#config-app-php