That's expected. The translator replaces placeholders as they are (i.e. in your case the call needs to be $translator->trans('product.add.success', ['{id}' => 1234]);). Note that there is not special meaning of the curly braces in regard to placeholders. You could also use the percent sign as often used too (i.e. having a message like 'Le produit %id% a été ajouté!' and then call the translator this way: $translator->trans('product.add.success', ['%id%' => 1234]);).

So I am going to close here as there is nothing to fix. Thank you for understanding.

Note that curly braces do have a special meaning when using the new ICU format. But for that, you would have to put your translation in messages+intl-icu.fr.yaml instead, to opt-in for the new format.

It is very strange behavior. Because in doc we see:

// translations/messages+intl-icu.en.php
return [
    'say_hello' => "Hello {name}!",
];

// prints "Hello Fabien!"
echo $translator->trans('say_hello', ['name' => 'Fabien']);

in fact we get Hello {Fabien}!
Moving on.

// translations/messages+intl-icu.en.php
return [
    'say_hello' => "You name {name}!",
];

echo $translator->trans('say_hello', ['name' => 'Fabien']);

and we get
You Fabien {Fabien}!