Create a custom entity type and define the form display as non-configurable in the base field definitions and also have the language module installed.
You'll expect that the language component is hidden in the form display, but it is not because of language_entity_base_field_info_alter() setting it to configurable without any other checks.
The only way to deactivate it would be to implement the same hook in the module where the entity type is being defined and revert what the hook from the language module has done, which isn't DX-friendly and also requires debugging or present knowledge of what the language module is doing.
Let language_entity_base_field_info_alter() set the displays of the language field configurable only if there is no value yet provided.
Needs work
10.1 ✨
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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Moving back to NW for the tests as those would still need to happen.