SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '5898', '1230', '1449', '1201', '1219', 
    '5899', '1269', '1283', '5900', '1281', 
    '5901', '1255', '1257', '5902', '5903', 
    '5913', '5904', '5905', '5906', '5907', 
    '5908', '1282', '1425', '1313', '1280', 
    '1254', '5909', '1314', '1200', '5910', 
    '5912', '1198', '1203', '1199', '5914', 
    '1241', '5915', '5916', '5940', '1204', 
    '5942'
  ) 
  AND lang_code = 'ru'

Query time 0.00890

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variant_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 41,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('5898','1230','1449','1201','1219','5899','1269','1283','5900','1281','5901','1255','1257','5902','5903','5913','5904','5905','5906','5907','5908','1282','1425','1313','1280','1254','5909','1314','1200','5910','5912','1198','1203','1199','5914','1241','5915','5916','5940','1204','5942') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}