[best practice] i18n.rst(Internationalization) を翻訳

originlal: https://github.com/symfony/symfony-docs/blob/master/best_practices/i18n.rst

* [x] i18n.rst #317の積み残し
```
But since the ``app/Resources/`` directory is considered the global location
for the application's resources, storing translations in ``app/Resources/translations/``
centralizes them *and* gives them priority over any other translation file.
This lets you override translations defined in third-party bundles.
```
これ先頭の`But`は不要でしょうか。
`is considered` 「は扱われる」と意訳しました。