Changed default value for locale to BCP-47 compatible

[darkdef]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌

[what-the-diff[bot]]

• Changed the default locale from en_US to en-US
• Updated tests for new format of locales

[codecov[bot]]

Codecov Report

Base: 95.25% // Head: 95.25% // No change to project coverage :thumbsup:

Coverage data is based on head (0f5e8f6) compared to base (ca4e597). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #110 +/- ## ========================================= Coverage 95.25% 95.25% Complexity 110 110 ========================================= Files 15 15 Lines 316 316 ========================================= Hits 301 301 Misses 15 15 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/translator/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Translator.php](https://codecov.io/gh/yiisoft/translator/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1RyYW5zbGF0b3IucGhw) | `90.00% <ø> (ø)` | | | [config/params.php](https://codecov.io/gh/yiisoft/translator/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-Y29uZmlnL3BhcmFtcy5waHA=) | `100.00% <100.00%> (ø)` | | | [src/SimpleMessageFormatter.php](https://codecov.io/gh/yiisoft/translator/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1NpbXBsZU1lc3NhZ2VGb3JtYXR0ZXIucGhw) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

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