While we don't support translating Content Types in the new Admin UI yet, saving these migrated from Legacy should at least be possible.

Steps to reproduce:

1. Install eZ Platform clean
2. Add via Admin UI another language: pol-PL
3. Create using PAPI new Content Type in a pol-PL language:

           $contentTypeGroup = $contentTypeService->loadContentTypeGroupByIdentifier('Content');
   
           $contentTypeStruct = $this->contentTypeService->newContentTypeCreateStruct('test_t');
           $contentTypeStruct->mainLanguageCode = 'pol-PL';
           $contentTypeStruct->names = ['pol-PL' => 'PL Test'];
   
           $fieldDefinitionCreateStruct = $contentTypeService->newFieldDefinitionCreateStruct('name', 'ezstring');
           $fieldDefinitionCreateStruct->position = 1;
           $fieldDefinitionCreateStruct->names = ['pol-PL' => 'Name'];
           $contentTypeStruct->addFieldDefinition($fieldDefinitionCreateStruct);
   
           $contentTypeDraft = $contentTypeService->createContentType($contentTypeStruct, [$contentTypeGroup]);
   
           $contentTypeService->publishContentTypeDraft($contentTypeDraft);
   

4. Try to edit that Content Type in Admin UI
5. Observe that both Content Type and Field Def names are empty.
6. Fill-in empty names.
7. Save Content Type.
8. Observe either HTTP 500 error in prod mode or "Notice: undefined index pol-PL" in dev mode.

Expected result

It should be possible to save Content Type without error.

Workaround

It's worth to notice that the error comes from the other language (pol-PL) not being present in the prioritized languages list of admin SiteAccess. Once it gets added, the issue disappears.

However I don't think that not having a language in the list should be a blocker for saving Content Type.