Improve country selection

- Reload countries if the current locale has changed in CountryList helper
- Add CountryList::getCountriesForLanguage
- In multilingual setup page allow selecting locales not associated to countries (eg: 'en' instead of 'en_US')
- In multilingual setup page suggest a list of countries, where a language is spoken (sorted by the people speaking that language).

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Turns this:

![pre](https://cloud.githubusercontent.com/assets/928116/5536493/1176448a-8a93-11e4-8f1f-b0accc7a6fa2.png)

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Into this:

![post](https://cloud.githubusercontent.com/assets/928116/5536494/1dd0638c-8a93-11e4-9a7b-fe0a2667157e.png)