[5.x] Fix appended form config fields when user locale differs from app locale

[duncanmcclean]

This pull request fixes an issue with appended form config fields when the user's locale differs from the application's locale.

For example: if you append fields to the existing "Fields" section, and a user is viewing the Control Panel in German, your appended fields would show but the existing fields wouldn't.

\Statamic\Facades\Form::appendConfigFields('*', 'Fields', [
    'a' => ['type' => 'text', 'display' => 'First injected into fields section'],
    'b' => ['type' => 'text', 'display' => 'Second injected into fields section'],
]);

This pull request fixes it by translating the section's display in the fields merging logic.

Fixes #11672.