Steps:

1. Installed any translation, for example Polish:

composer require ezplatform-i18n/ezplatform-i18n-pl_pl

2. Set up Polish language in web browser
3. Create new file in vendor/ezplatform-i18n/ezplatform-i18n-pl_pl/ezplatform-admin-ui/messages.pl_PL.xlf (it can be copied from ezplatform-admin-ui/src/bundle/Resources/translations/messages.en.xliff)/
4. Edit messages.pl_PL.xlf and change any target.
5. Run

composer run post-update-cmd

Actual result:

Nothing is translated. Symfony profiler displays locale en_GB. After logging in everything is translated.

Expected result:

User should see translated phrases.