Use i18n when possible

[dergachev]

Docs (this page seems still valid, despite labeled being out of date and never updated for Vagrant 1.2+): http://docs-v1.vagrantup.com/v1/docs/extending/i18n.html

Also see this as an example:

• https://github.com/scalefactory/vagrant-multiprovider-snap

It seems that everyone (even vagrant itself) doesn't bother translating command help and options:

• https://github.com/mitchellh/vagrant/blob/master/plugins/commands/plugin/command/list.rb
• https://github.com/mitchellh/vagrant/blob/master/plugins/commands/plugin/action/list_plugins.rb#L42