The language detection method (visibility settings) always use language of the interface text. Switching to content language is not possible, even there is s select-box for.
1. Edit a custom markup block.
2. Unser Visibility settings switch to "Content".
3. Save and edit block again. You will find the settings switched back to "Interface language"
Make sure the selected option is safed and used.
Closed: cannot reproduce
1.1
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
In a standard installation with the language module enabled, only the first part of the form is shown:
Language selection [ ] English [ ] German Select languages to enforce. If none are selected, all languages will be allowed.
The language selector seems to be added by some third-party extension. Any way, it is not related to the custom markup block itself.