Allow `locale` key to be used in `config ...` commands. (#1952)

* Allow 'locale'key to be used in 'config ...' commands

* Fix documentation

* Update docs/configuration.md
Co-authored-by: per1234 <accounts@perglass.com>

* Update docs/configuration.md
Co-authored-by: per1234 <accounts@perglass.com>
Co-authored-by: per1234 <accounts@perglass.com>

cli/config/validate.go

@@ -33,6 +33,7 @@ var validMap = map[string]reflect.Kind{
 	"directories.builtin.tools":     reflect.String,
 	"directories.builtin.libraries": reflect.String,
 	"library.enable_unsafe_install": reflect.Bool,
+	"locale":                        reflect.String,
 	"logging.file":                  reflect.String,
 	"logging.format":                reflect.String,
 	"logging.level":                 reflect.String,

docs/configuration.md

@@ -18,6 +18,8 @@
   - `enable_unsafe_install` - set to `true` to enable the use of the `--git-url` and `--zip-file` flags with
     [`arduino-cli lib install`][arduino cli lib install]. These are considered "unsafe" installation methods because
     they allow installing files that have not passed through the Library Manager submission process.
+- `locale` - the language used by Arduino CLI to communicate to the user, the parameter is the language identifier in
+  the standard POSIX format `<language>[_<TERRITORY>[.<encoding>]]` (for example `it` or `it_IT`, or `it_IT.UTF-8`).
 - `logging` - configuration options for Arduino CLI's logs.
   - `file` - path to the file where logs will be written.
   - `format` - output format for the logs. Allowed values are `text` or `json`.