yesod-recaptcha2

import Yesod.ReCaptcha2

instance YesodReCaptcha App where
    reCaptchaSiteKey = pure "foo"
    reCaptchaSecretKey = pure "bar"
    reCaptchaLanguage = pure Nothing

    -- with specific language from https://developers.google.com/recaptcha/docs/language
    -- reCaptchaLanguage = pure (Just "ru")

Append to applicative form

buildForm :: Form MyForm
buildForm = renderDivs $ MyForm
  <$> areq textField "foo" Nothing
  <* reCaptcha