Description

In 4.1 doc, we had https://symfony.com/doc/4.1/components/translation/usage.html#explicit-interval-pluralization, it was removed in 4.2

Actual official recommendation is using ICU format

Since we have Translation Providers in core, there is a new global issue : it's about to not lose the ICU information when we push/pull translation to/from a Provider.

When we push, it's ok most of the time (but we can not be sure that some Provider will expect us to explicit the format used by pushed translations).
The main issue is when we pull translations from a Provider, we can not guess if received translations are in ICU format or not (most of the Providers expose an option to force the export format of translations, but we do not use them for now).

What do you think to deprecate this old format by introduction of a brand-new-already-deprecated config option legacy_translation_format (I'm not sure about the default value to avoid big BC) to allow users to use the old format until the next major version of Symfony? With this option, we could detect the usage of ICU or not in an application and keep this information to be used in Translation Providers read method.

And in the next major version, we will remove this config option, and all the code related to the old format, and finally the suffix in translation filenames.