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 = 'en' 
WHERE 
  l.layout_id = 1 
  AND l.dispatch IN ('categories.view') 
  AND (
    FIND_IN_SET(266, l.object_ids) 
    OR l.object_ids = ''
  ) 
ORDER BY 
  l.object_ids desc 
LIMIT 
  1

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "table": {
          "table_name": "l",
          "access_type": "ALL",
          "rows": 22,
          "filtered": 100,
          "attached_condition": "((l.layout_id = 1) and (l.dispatch = 'categories.view') and (find_in_set(266,l.object_ids) or (l.object_ids = '')))"
        }
      }
    },
    "table": {
      "table_name": "d",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "10",
      "used_key_parts": ["location_id", "lang_code"],
      "ref": ["egkcysbp_cscartn.l.location_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(((d.location_id = l.location_id) and (d.lang_code = 'en')))"
    }
  }
}