i18n2csv.rb

require 'csv'

def to_chained_key(result, hash, prefix = nil)
  hash.each do |key, value|
    current_key = [prefix, key].compact.join('.')

    if value.is_a?(Hash)
      to_chained_key(result, value, current_key)
    else
      result[current_key] = value
    end
  end

  result
end

I18n.t('raise exception') rescue nil # Load translations
translations = I18n.backend.instance_variable_get(:@translations)
result = to_chained_key({}, translations[:ja])
invalid_locales = {}
headers = ['key', 'ja', 'en', '...']

CSV.open('i18n.csv', 'w', col_sep: "\t", write_headers: true, headers: headers) do |csv|
  result.each do |key, value|
    case value
    when String, TrueClass, FalseClass
      csv << [key, value]
    when NilClass
      csv << [key, nil]
    when Array
      invalid_locales[key] = value
    else
      raise 'unsupported'
    end
  end
end

puts "下記は別途対応"
puts invalid_locales