TYPO3 Core

• How are both tables configured TCA-wise? It is difficult to guess how it's done...
• Which field should/might be used in order to determine that assignments with uids 23 and 24 actually are belonging together?

TCA Project:

return [
    'ctrl' => [
        'title' => 'project',
        'label' => 'name',
        'tstamp' => 'tstamp',
        'crdate' => 'crdate',
        'cruser_id' => 'cruser_id',
        'languageField' => 'sys_language_uid',
        'transOrigPointerField' => 'l10n_parent',
        'transOrigDiffSourceField' => 'l10n_diffsource',
        'delete' => 'deleted',
        'enablecolumns' => [
            'disabled' => 'hidden',
            'starttime' => 'starttime',
            'endtime' => 'endtime',
        ],
        'default_sortby' => 'ORDER BY name',
        'searchFields' => '',
        'iconfile' => 'EXT:ext/Resources/Public/Icons/Tables/project.svg',
    ],
...
    'columns' => [
        'site_assignment' => [
            'exclude' => false,
            'l10n_mode' => 'exclude',
            'label' => 'assignment',
            'config' => [
                'type' => 'inline',
                'foreign_table' => 'tx_ext_domain_model_projectsiteassignment',
                'foreign_field' => 'project',
                'appearance' => [
                    'collapseAll' => true,
                    'newRecordLinkAddTitle' => false,
                ],
            ],
        ],
...
    ],
];

TCA Table assignment:

return [
    'ctrl' => [
        'title'    => 'projectsiteassignment',
        'label' => 'category',
        'tstamp' => 'tstamp',
        'crdate' => 'crdate',
        'cruser_id' => 'cruser_id',
        'iconfile' => 'EXT:ext/Resources/Public/Icons/Tables/project.svg'
    ],
...
    'columns' => [
        'project' => [
            'exclude' => false,
            'label' => 'project',
            'config' => [
                'type' => 'select',
                'renderType' => 'selectSingle',
                'foreign_table' => 'tx_ext_domain_model_project',
                'foreign_table_where' => 'AND tx_ext_domain_model_project.pid=###CURRENT_PID### AND tx_ext_domain_model_project.sys_language_uid IN (-1,0) ORDER BY tx_ext_domain_model_project.name',
                'enableMultiSelectFilterTextfield' => true,
                'minitems' => 1,
                'maxitems' => 1,
            ],
        ],
        'prestige_reference' => [
            'exclude' => false,
            'label' => 'prestige_reference',
            'config' => [
                'type' => 'check',
                'items' => [
                    '1' => [
                        '0' => 'LLL:EXT:lang/Resources/Private/Language/locallang_core.xlf:labels.enabled'
                    ]
                ],
                'default' => 0
            ]
        ],
...
    ],

There is no need to have a relation between the assignment records! Those created for the translation of a project are only generated because the core does this ("record unrolling" how I like to call it). This is not technically needed for the use case, where it would suffice to store the assignment once for a project (no matter how it is translated).

In terms of "record unrolling" the core behaviour is clear and okay (even though it wastes some uids in DB due to the delete and re-create), but there is no other possibility to keep the things in sync. But, the sync has to happen after the assignments in the original language have been persisted not before.

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/62871