SELECT 
  variant_descriptions.variant 
FROM 
  cscart_product_features_values AS v 
  LEFT JOIN cscart_product_feature_variant_descriptions AS variant_descriptions ON variant_descriptions.variant_id = v.variant_id 
  AND v.lang_code = variant_descriptions.lang_code 
WHERE 
  v.product_id = 276708 
  AND v.feature_id = 18 
  AND variant_descriptions.lang_code = 'en'

Query time 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.38"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "v",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "variant_id",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "fl",
          "used_key_parts": [
            "feature_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "const",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "5.00",
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.01",
            "prefix_cost": "0.35",
            "data_read_per_join": "39"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "(`s2cart`.`v`.`product_id` = 276708)"
        }
      },
      {
        "table": {
          "table_name": "variant_descriptions",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "s2cart.v.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "100.00",
          "index_condition": "(`s2cart`.`variant_descriptions`.`lang_code` = `s2cart`.`v`.`lang_code`)",
          "cost_info": {
            "read_cost": "0.02",
            "eval_cost": "0.01",
            "prefix_cost": "0.38",
            "data_read_per_join": "308"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}