applocales.rb

require 'yaml'

class LocaleGroup
  attr_reader :output

  def initialize
    @output = {}
    @output.default_proc = -> (h, k) { h[k] = Hash.new(&h.default_proc) }
  end

  def to_yaml
    output.to_yaml
  end

  def nest(key, value)
    array = key.split(".")
    output.dig(*array[0..-2])[array.fetch(-1)] = value
  end
end

locales = LocaleGroup.new

locales.nest("en.whatever.devise.login.welcome", "Hey")
locales.nest("en.whatever.devise.login.hello", "Hello")
locales.nest("en.yolo", "Dolo")

puts locales.to_yaml

output.yaml

---
en:
  whatever:
    devise:
      login:
        welcome: Hey
        hello: Hello
  yolo: Dolo