add app.setLocale() (#11469)

* infrastructure for setLocale via klang

* add documentation for setLocale

* add test for setLocale

* fix spec

* add spec and update docs

* fix carriage feeds on windows

* SetLocale() sets LC_ALL on Linux

* in SetLocale() on Linux, use g_setenv()

* fix tyop: '#ifdef OSX_POSIX'

* make the linter happy

* improvements from review

docs/api/app.md

@@ -530,6 +530,12 @@ Returns `String` - The current application locale. Possible return values are do
 
 **Note:** On Windows you have to call it after the `ready` events gets emitted.
 
+### `app.setLocale(locale)`
+
+* `locale` String
+
+Set the locale of the app (must be called before the `ready` event).
+
 ### `app.addRecentDocument(path)` _macOS_ _Windows_
 
 * `path` String