Add support for custom prompt per locale.

[pcreux]

Add support for a custom prompt per locale, allowing us to define glossaries.

Ex:

# config/i18n-tasks.yml
translation:
  # ...
  openai_locale_prompt_extras:
    fr: >-
      "Field" should translate to "Magasin" (as in a retail store).
      "Field" can be translated as "Champ" when referring to a form field.
      "Learning" should be translated as "Formation".
      "Tracks" should translate to "Parcours" (as in a learning track).
      "Insights" should translate to "Indicateurs".
  it: >-
      "Field" should translate to "Negozio" (as in a retail store).
      "Learning" should be translated as "Formazione".
      "Tracks" should translate to "Percosi" (as in a learning track).
      "Insights" should translate to "Reports".

[markedmondson]

No test?! :)