Файлы перевода Symfony 4 в каталогах

Translation Resource/File Names and Locations

Symfony looks for message files (i.e. translations) in the following default locations:

• the translations/ directory (at the root of the project);
• the Resources/translations/ directory inside of any bundle.

The locations are listed here with the highest priority first. That is, you can override the translation messages of a bundle in any of the top two directories.

The override mechanism works at a key level: only the overridden keys need to be listed in a higher priority message file. When a key is not found in a message file, the translator will automatically fall back to the lower priority message files.

Источник: https://symfony.com/doc/4.2/translation.html#translation-resource-file-names-and-locations

И далее на той же странице/главе

You can add other directories with the paths option in the configuration:

config/packages/translation.yaml

framework:
    translator:
        paths:
            - '%kernel.project_dir%/translations/admin'
            - '%kernel.project_dir%/translations/client'

Вы можете использовать их как любые другие файлы перевода, помня механизм переопределения, указанный здесь выше.

Полная ссылка: https://symfony.com/doc/4.2/translation.html