Provide a getDefaultLocale in I18NProvider

[jcgueriaud1]

Describe your motivation

Currently the default locale is calculated in LocaleUtil.
It's the first locale in the list.

With our default provider, it's the first in alphabetical order. For example if your application is in German (de) and English then the default in German.

To change the default you need to change the order of the list

Describe the solution you'd like

Provide a getDefaultLocale() in I18NProvider that returns by default the first locale.

That makes the override easy.