SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'ja' 
WHERE 
  pfv.variant_id IN (18, 19, 20)

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfv",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 3,
      "filtered": 100,
      "index_condition": "pfv.variant_id in (18,19,20)"
    },
    "table": {
      "table_name": "pfvd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "ref": ["cscart_purchase_fl_com.pfv.variant_id", "const"],
      "rows": 1,
      "filtered": 100,
      "index_condition": "pfvd.lang_code = 'ja'"
    }
  }
}