If you want to translate a field in multiple locales with a single form you can use the overridden fill() method which allows you to pass in an array with the locales as first array key and the translated attributes in the sub-array.

'en' => [
'title' => 'My first edited post',
'de' => [
'title' => 'Mein erster bearbeiteter Beitrag',

To achieve this structure in your form - to prevent manipulating the form data just to save them. You can use the input name array [] syntax.

<input type="text" name="en[title]" />
<input type="text" name="de[title]" />