Use xx-XX format for translation files and default to en-US

Many languages have different variants, so instead of "en" we should be
using "en-US" - this will make it possible to add slightly different
British English translations, for example.
Note that this was already supported for the `locale` config option, we
were simply discarding the part after the dash when looking for the
right translation file.

Also make sure the en-US translations are actually American English,
I'll add en-GB separately.

docs/config.md

@@ -171,15 +171,16 @@ formats.
 
 Available languages:
 
-- `de` (German)
-- `en` (English)
-- `es` (Spanish)
-- `fr` (French)
+- `de-DE` (Standard German)
+- `en-US` (American English)
+- `es-ES` (Castilian Spanish)
+- `fr-FR` (Standard French)
 
-You can use formats like `en-GB`, `en-US`, `de-DE`, `fr-FR`.
+Using a locale with non-existent translations is possible and will affect date/time formats, but
+use `en-US` for translations.
 
 - Type: [`String`]
-- Default: `"en"`
+- Default: `"en-US"`
 
 ### `map.attribution`