SELECT 
  * 
FROM 
  cscart_bm_locations as l 
  LEFT JOIN cscart_bm_locations_descriptions as d ON d.location_id = l.location_id 
  AND d.lang_code = 'ja' 
WHERE 
  l.layout_id = 6 
  AND l.dispatch IN ('categories.view') 
  AND (
    FIND_IN_SET(5, l.object_ids) 
    OR l.object_ids = ''
  ) 
ORDER BY 
  l.object_ids desc 
LIMIT 
  1

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "ordering_operation": {
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "l",
            "access_type": "ALL",
            "rows": 134,
            "filtered": 100,
            "attached_condition": "((`_testcscart`.`l`.`layout_id` = 6) and (`_testcscart`.`l`.`dispatch` = 'categories.view') and (find_in_set(5,`_testcscart`.`l`.`object_ids`) or (`_testcscart`.`l`.`object_ids` = '')))"
          }
        },
        {
          "table": {
            "table_name": "d",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "location_id",
              "lang_code"
            ],
            "key_length": "10",
            "ref": [
              "_testcscart.l.location_id",
              "const"
            ],
            "rows": 1,
            "filtered": 100,
            "attached_condition": "<if>(is_not_null_compl(d), ((`_testcscart`.`d`.`location_id` = `_testcscart`.`l`.`location_id`) and (`_testcscart`.`d`.`lang_code` = 'ja')), true)"
          }
        }
      ]
    }
  }
}