SELECT 
  pfvd.variant 
FROM 
  cscart_product_features_values AS pfv 
  LEFT JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfvd.variant_id = pfv.variant_id 
  AND pfvd.lang_code = 'ko' 
WHERE 
  pfv.feature_id = '2' 
  AND pfv.product_id = 25284

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfv",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "idx_product_feature_variant_id",
      "key_length": "6",
      "used_key_parts": ["product_id", "feature_id"],
      "ref": ["const", "const"],
      "rows": 11,
      "filtered": 100,
      "using_index": true
    },
    "table": {
      "table_name": "pfvd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "ref": ["devmotorsbay.pfv.variant_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(pfvd.lang_code = 'ko')"
    }
  }
}