Fixes #2079:

- 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`
Merge branch 'master'

Conflicts:
	framework/CHANGELOG.md
- Fixed BaseFileHelper::localize docs. - DbMessageSource now performs UNION ALL. - Fixed message merging for php and po. - Updated tests.