When internationalizing your Rails app, you'll be replacing strings like 'Please enter your name' with t('.name_prompt'). You will be adding keys to your config/locales/*.yml files over and over again. Not to miss any key and place each at the right place is a challenging task.
The gem i18n-tasks has you covered. See 
  its README
  
    Show archive.org snapshot
  
 for a list of things it will do for you.
Note
The
i18n-tasksgem does not understand aliases and will duplicate all referenced data when it writes locales. If you are using YAML anchors, aliases and merge keys as suggested in this card, you should disable auto-insertion of missing keys (by having an emptywrite:key inconfig/i18n-tasks.yml. Since you need to manually put in the actual translation anyway, this is no drawback.
Test that I18n keys are translated into every locale
This spec will fail if a translaton key is missing:
require 'i18n/tasks'
describe 'I18n' do
  let(:i18n) { I18n::Tasks::BaseTask.new }
  let(:missing_keys) { i18n.missing_keys }
  let(:unused_keys) { i18n.unused_keys }
  it 'does not have missing keys' do
    expect(missing_keys).to be_empty,
      "Missing #{missing_keys.leaves.count} i18n keys, run `i18n-tasks missing' to show them"
  end
  it 'does not have unused keys' do
    expect(unused_keys).to be_empty,
      "Missing #{unused_keys.leaves.count} i18n keys, run `i18n-tasks unused' to show them"
  end
end
Alternatives
Simple
"Whenever I18n::MissingTranslationData exception is caught, 
  i18n_translation_spawner
  
    Show archive.org snapshot
  
 tries to add a new key into your translation file, generates new translation for it and does it within all locales you have defined in your app."
Extended
The plugin 
  translate
  
    Show archive.org snapshot
  
 offers a nice web interface for manual translation, presenting you with a easy-to-use list of all your i18n keys.
Caveat: Will reformat your .yml file and use no quotation marks.
Most powerful
DHH's plugin 
  tolk
  
    Show archive.org snapshot
  
 needs an own database table but is the most powerful of these tools.
Know that this is only a small collection, there are many more tools.