Part 1

• in any unit config, that has associated database table add following to the Fields array:

'TranslateFrom' => Array (
  'type' => 'int',
  'formatter' => 'kMultiLanguage', 'db_type' => 'int', 'index_type' => 'int',
  'not_null' => 1, 'default' => 0
),

• make sure Debug mode is enabled and DBG_REDIRECT option is turned on
• press Rebuild button in Rebuild Multilingual Fields sub-section in Tools → System Tools section
• confirm, that Undefined index ... notice from multilanguage_helper.php isn't happening
• confirm, that ADD INDEX(TranslateFrom) was used in ALTER statement in Debugger Report

Part 2

• in any unit config, that has associated database table add following to the Fields array:

'TranslateFrom' => Array (
  'type' => 'string',
  'formatter' => 'kMultiLanguage', 'db_type' => 'varchar(255)', 'index_type' => 'string',
  'not_null' => 1, 'default' => ''
),

• make sure Debug mode is enabled and DBG_REDIRECT option is turned on
• press Rebuild button in Rebuild Multilingual Fields sub-section in Tools → System Tools section
• confirm, that Undefined index ... notice from multilanguage_helper.php isn't happening
• confirm, that ADD INDEX(TranslateFrom (5)) was used in ALTER statement in Debugger Report

Part 3

• in any unit config, that has associated database table add following to the Fields array:

'TranslateFrom' => Array (
  'type' => 'string',
  'formatter' => 'kMultiLanguage', 'db_type' => 'varchar(255)',
  'not_null' => 1, 'default' => ''
),

• make sure Debug mode is enabled and DBG_REDIRECT option is turned on
• press Rebuild button in Rebuild Multilingual Fields sub-section in Tools → System Tools section
• confirm, that Undefined index ... notice from multilanguage_helper.php isn't happening
• confirm, that ADD INDEX(TranslateFrom (5)) was used in ALTER statement in Debugger Report