Currently the "Language" field on multilingual-enabled content does not have a default when you create content. Should this field carry over the default language from the site configuration, preselecting the default? Or should there be settings on the field per content type, for example, to set a default for that type of content? It would be helpful to be able to specfiy whether this field is required or not as well.

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Hmmm... this sounds like a bug with Copy Blocks. Would this possibly be related: https://github.com/backdrop-contrib/copy_blocks/issues/26 https://github.com/backdrop-contrib/...
missing "add block" in custom layout
Also this error: Warning: Undefined array key "blog" in summary() (line 63 of /app/core/modules/layout/plugins/access/entity_bundle_layout_access.inc). I did recently delete an old blog...
missing "add block" in custom layout
I don't see any errors in the browser console, but there's this error in the logs Warning: Trying to access array offset on null in summary() (line 63 of /app/core/modules/layout/plugins/...
missing "add block" in custom layout