Performance wise, it may be better to simply provide the fields and use the content if they are completed. This means only one if statement and one or two requests depending on if the first returns something.
On the other hands, see the WP doc on get_post_meta() and calling it without specifying a meta key.
For usability, it would likely be better to provide a checkbox field for the user to select the other languages they want to translate to. This would meant you’d always need to perform at least to requests and possibly a 3rd.
1 – see if translation is on for a language
2 – ? Get the content for the language if it’s selected
3 – ? Get the default language if translation is not selected or if 2 returns no value
