SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '3719', '3709', '4862', '1913', '4867', 
    '1858', '1882', '1855', '2178', '2547', 
    '1880', '1910', '2128', '1288', '176', 
    '1856', '1983', '1859', '1870', '360', 
    '1293', '1349', '1290', '1447', '1376', 
    '860', '491', '1338', '1291', '1410', 
    '1402', 'Y'
  ) 
  AND lang_code = 'ru'

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 32,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('3719','3709','4862','1913','4867','1858','1882','1855','2178','2547','1880','1910','2128','1288','176','1856','1983','1859','1870','360','1293','1349','1290','1447','1376','860','491','1338','1291','1410','1402','Y') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}