title: How to validate that an object property is required?
body: <p>I have object types that have a 'translations' property where the fields that can be translated into different languages are passed into the specific 'lang' property as you can see in the schema below.</p>
  <p>An English translation is always required and the rest of the languages are optional, I can achieve this by setting <code>.default(undefined)</code> to the optional languages.</p>
  <p>When a language is present and the validation for its inner fields fails, the error is always associated to the field itself ('name' in this case). This behaviour is expected.</p>
  <p>What I else want to achieve and I don't know how is to show an error when the 'translations' property 'en' is not present with a message like <code>'An English translation is required'</code>.</p>
  <pre><code>const categoryTranslationsSchema = object({
      name: string()
          .min(3, 'Must have at least 3 characters.')
          .max(16, 'Cannot be longer than 16 characteres.')
          .required('Must provide a name.')
  })
  
  const categorySchema = object({
      slug: string()
          .min(3, 'Must have at least 3 characters.')
          .max(16, 'Cannot be longer than 16 characteres.')
          .lowercase()
          .trim()
          .matches(/^(?![0-9-]+$)(?:[a-z]{2,}-?|[0-9]-?)+(?&lt;!-)$/gm, 'Must start with a letter and can'
              + ' only contain letters, numbers or dashes (no more than one consecutive).')
          .required('The slug is required.'),
      translations: object({
          en: categoryTranslationsSchema,
          es: categoryTranslationsSchema
              .default(undefined),
          de: categoryTranslationsSchema
              .default(undefined)
      })
  })
  </code></pre>