SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '2526', '1892', '1855', '1930', '3726', 
    '47', '462', '463', '501', '1679', 
    '126', '1457', '635', '127', '122', 
    '1680', '123', '3587', '1631', '535', 
    '421', '437', '422', '129', '49', '124', 
    '66', '616', '88', '50', '128', '2982', 
    '1856', '1870', 'Y', '2986', '136', 
    '91', '65', '2987'
  ) 
  AND lang_code = 'ru'

Query time 0.00112

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": 40,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('2526','1892','1855','1930','3726','47','462','463','501','1679','126','1457','635','127','122','1680','123','3587','1631','535','421','437','422','129','49','124','66','616','88','50','128','2982','1856','1870','Y','2986','136','91','65','2987') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}