TYPO3 Core

The TCA file should be extended to setup table indexes.

1. option

  'types' => [
  ...
  ],
  'columns' => [
  ...
  ],
  'indexes' => [
    'language_identifier' => ['l10n_parent','sys_language_uid'],
    'mn_key' => ['key_to_other_table'],
    ...
  ],

This would leave more room to also include fields set by Typo3 like pid or uid.

or

2. option

  'columns' => [
    'parent_key' => [
      'config' => [
         ...
        'foreign_table' => 'tx_extname_domain_model_parent',
        'index' => 'index_name',
      ],
    ],
    ...
  ],

With the 2. option one would just set the index by the field and if more than one field have the same index name it becomes an index over the all fields.

Maybe having both options could be useful?

As this is just the idea I have not thought about index types like primary, unique, ... or order.