[WIP] Update translation support

<table>
  <tr>
    <th>Q</th><th>A</th>
  </tr>
  <tr>
    <td>Bug fix?</td><td>no</td>
  </tr>
  <tr>
    <td>New feature?</td><td>no</td>
  </tr>
  <tr>
    <td>BC breaks?</td><td>no</td>
  </tr>
  <tr>
    <td>Tests pass?</td><td>yes</td>
  </tr>
  <tr>
    <td>License?</td><td>MIT</td>
  </tr>
</table>

- [x] Tweak translation messages
- [x] Fix default locale on the configuration (maybe useless)
- [x] Add messages loader
- [x] Update the template helper
- [x] Add tests

PS: need to clean commits before merging.