In order to reduce the stack image size, the heroku 16 stack doesn't include language packs by default.
You may still want to use a custom language pack though.
heroku buildpacks:add heroku-community/locale
Create a .locales file at your app's root, with all the locales you need. Example:
de_DE
fr_FR
Commit that file and trigger a new build. The specified locales will be installed by the buildpack.
You can test this buildpack in a local docker container with the following command:
make compile
Then, to connect to the container:
docker-compose run test bash