rg90 You can translate directly through Weblate, translations should be automatically exported to GitHub repository in about 1 hour, and then you should be able to install new translations using composer update flarum-lang/czech
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Currently, there is no maintainer for Czech language pack, but maintainer is not really necessary - community may stil translate through Weblate and their work will exported to language pack. But if you want to take care of this language pack, I could grant you necessary permissions.