As an editor I want to use Admin panel in my language, so that it's more accessible to me.

Similar as in v1, it should be possible to translate the Admin panel to one of the languages installed based on user's browser settings.

Some example scenarios:

Given a user with browser configured to fr
and ezplatform-i18n-fr_FR language package installed
When I open the admin panel
Then it's translated to French

Given a user with browser configured to fr_FR
and ezplatform-i18n-fr_FR language package installed
When I open the admin panel
Then it's translated to French

Given a user with browser configured to fr_FR
and ezplatform-i18n-pl_PL language package installed
When I open the admin panel
Then it's translated to English (default language)

Given a user with browser configured to fr, pl;q=0.7
and ezplatform-i18n-pl_PL language package installed
When I open the admin panel
Then it's translated to Polish

Given a user with browser configured to fr, pl;q=0.7
and ezplatform-i18n-fr_FR language package installed
and ezplatform-i18n-pl_PL language package installed
When I open the admin panel
Then it's translated to French

User settings should be based on Accept-Language header sent by the browser.