Update Language Settings Information

[Kagetsuki]

Spree::Config.default_locale no longer exists, and stale initializers will cause issues. The documentation should be updated to reflect this. Something like:

If you are upgrading from a previous version of Spree please be aware Spree::Config.default_locale is no longer used. Any initialzers trying to set it should be removed.

To set the default language simply set config.i18n.default_locale in config/application.rb to your desired language. EG:
config.i18n.default_locale = :ja

[runlevel5]

@Kagetsuki can you please submit a pull request?

[jay2u]

@Kagetsuki I'd be more than happy to rewrite the guides to reflect this. Can you get in contact with me to describe this phenomenon in more detail? I've researched the issue but need more info to be able to pinpoint the specifics.