According to the documentation and the translation:update description, php bin/console translation:update --force --dump-messages en is supposed to store translations in app/Resources by default.

However, in #25065 the defaultTransPath constructor argument was set to %kernel.project_dir%/translations instead of %kernel.project_dir%/app/Resources:

The former is indeed correct for sf4 projects, but directly contradicts the description of the command (and probably some documentation) in 3.4:

Usage:
  translation:update [options] [--] <locale> [<bundle>]

Arguments:
  locale                               The locale
  bundle                               The bundle name or directory where to load the messages, defaults to >>> app/Resources <<<  folder

php bin/console translation:update --force --dump-messages en app does store the translations in app/Resources/translations.

I really don't mind typing the 3 extra letters, but I thought I'd mention it either way :)
Also, not really sure if I missed anything here.