Content Translation integrates several Machine Translation (MT) services. Those services may provide support for English, but they do not provide specific support for Simple English.
Users of Simple English Wikipedia can benefit from the English MT as a starting point, but this is not always exposed to them. For example, when translating from Simple English to Spanish, both Apertium and Yandex support English-to-Spanish translations but only Apertium is shown as an option:
We should review the MT services that support English, and make sure that they are available as an option when using Simple English. This request is based on [[ https://www.mediawiki.org/wiki/Topic:Udmnnnrjpzq5dbe0 | this comment on the project talk page ]].
Please, when making these adjustments make sure for MT not to be enabled for English Wikipedia by accident.