New language: Basque (eu)

Someone opened a PR to add the Basque language: https://github.com/nuxeo/nuxeo-drive/pull/1715

Manon Lumeau already added the new language on Crowdin.

This ticket is just here to have an history, but some minor changes must be done on the repository:

• add the Basque language to the installer (Windows, see here)
• update https://github.com/nuxeo/nuxeo-drive/blob/master/docs/support.md#translations
• and also: it is a good opportunity to add a simple test on the Translator to list all available locales and check all are present and valid.