class Page
  include Mongoid::Document
  field :seo_title, type: String, default: "", localize: true
  validates_length_of :seo_title, within: 3..32, allow_blank: true
end

p = Page.new
=> #<Page _id: 51ee3e98de939d0721000003, seo_title: {"en"=>""}>
p.valid?
=> false
p.errors
#<ActiveModel::Errors:0x002b10782a3c40 @base=#<Page _id: 51ee3e98de939d0721000003, seo_title: {"en"=>""}>, @messages={:seo_title=>["is too short (minimum is 3 characters)"]}>