This is currently not possible with WPML. Once you have set any page as your Front or Post page on the Settings → Reading page, WPML will immediately assign their translations to secondary languages.
When you permanently delete translations of your front-page, your default language page loses its front-page status. This is expected behavior due to the way WordPress saves these settings to the database.
The options from Settings → Reading are saved in the wp_options table, which contains only one value. Deleting the original page or its translation from the A static page (select below) option also deletes the value from the database. As a result, the front-page settings reset back to Your latest posts.