By default, English strings are used in the bundles. We can make it configurable by specifying the language in build-config.js and reading it from Webpack config files.
An example of using our Webpack plugin is here: https://github.com/ckeditor/ckeditor5-labs/tree/master/integrations/webpack-localization
A lot of translation' files are missing:

Maybe true right now since we haven't regenerated them for a long time. I'll do it today.
But it works :D

The question is – should we add the CKEditorWebpackPlugin every time or only if the specified language is other than en?
I think all the time. To KISS and to make sure that the translations (even en->en) are used.
I can't upload the translations for now: https://github.com/ckeditor/ckeditor5-dev/issues/249.