Localization not applied

Summary
My Docker 11.3 Enterprise edition is set to French but my changes are not applied even after restart.
Close to this topic Changing display language has no effect - #5 by verallgemeinung

Steps to reproduce
Log in to MM on the web > console > site configuration > localization
set server and client language to anything else than English.
restart MM docker

Expected behavior
it should be in the target language

Observed behavior
stays in English. Using .env variables doesn’t improve

Any idea?