Symfony version(s) affected: 4.4.18

Description
See there for the reproduction project:
https://github.com/Ser5/SymfonyTranslatorBug

I want to use the Symfony validators library in standalone mode. Therefore, AFAIK, I should also use the translator library, but during experiments with it I found that it doesn't work as I expected.

According to these docs
https://symfony.com/doc/current/components/form.html
I installed symfony/config and symfony/translation packages via Composer.

According to these
https://symfony.com/doc/current/translation/message_format.html
created messages file and index.php file.

Then ran index.php.
expected:

Hello Fabien!
Hello Symfony!

got:

Hello {Fabien}!
Hello {Symfony}!

How to reproduce

git clone https://github.com/Ser5/SymfonyTranslatorBug.git
cd SymfonyTranslatorBug/lib/
composer install
cd ../
php index.php

Possible Solution
May be one of these problems:

• I have read the docs wrong - where?
• Docs are unclear - clear them
• Docs are outdated - update them
• Libs are buggy - fix them

Additional context
Windows 10
PHP 7.2.4