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.
Merge pull request #2119 from yiisoft/i18n-language-fallback

Fixes #2079