'The localization demo has a custom locale (German, custom.locale.De.js). You can inspect it and the demo to see how to create your own and how to include it.'
where is 'custom.locale.De.js' ??
BR
Please check examples/localization/locales
folder in the sources you downloaded in CustomerZone.
It's noted here https://bryntum.com/products/scheduler/docs/guide/Scheduler/customization/localization#create-a-custom-locale
All the best,
Alex