Add translation for custom field

You can simply use the translated string directly without _e()
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But I want it to function in both languages like the rest of the template does.
We use the frontend in English and the backend in another language.

I see, for custom strings you’ll have to use your child theme or theme translations, which have their own textdomain. You can do that via Loco Translate too, and then simply use that textdomain in the _e() function (using ‘your-child-theme’ as the domain in this example):

_e( 'Custom text', 'your-child-theme' );