Fixes #3687: Default `sourceLanguage` and `language` are now `en-US` in order for i18n formatter to work correctly

issue #2852

Fixes #2527: Source language for `app` message category was always `en` no matter which application `sourceLanguage` was used

Fixes #2157: The '*' category pattern will match all categories that do not match any other patterns listed in `I18N::translations`

- i18n now falls back to `en` from `en-US` if message translation isn't found
- View now falls back to `en` from `en-US` if file not found
- Default `sourceLanguage` and `language` are now `en`

Fixes #1654: Fixed the issue that a new message source object is generated for every new message being translated

- added example code for application components
- added missing short description

[ci skip]

they are not responsible for i18n ;)

[ci skip]

fixes #991