SELECT 
  c.parent_id, 
  sm.search_phrases 
FROM 
  cscart_categories AS c 
  LEFT JOIN cscart_ab__search_motivation AS sm ON sm.category_id = c.category_id 
  AND sm.lang_code = 'en' 
WHERE 
  c.category_id = 951

Query time 0.00024

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "c",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY",
            "p_category_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "category_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.10",
            "prefix_cost": "0.00",
            "data_read_per_join": "3K"
          },
          "used_columns": [
            "category_id",
            "parent_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "sm",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "category_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "index_condition": "<if>(is_not_null_compl(sm), (`s2cart`.`sm`.`lang_code` = 'en'), true)",
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.35",
            "data_read_per_join": "32"
          },
          "used_columns": [
            "category_id",
            "lang_code",
            "search_phrases"
          ]
        }
      }
    ]
  }
}