SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '2998', '4796', '4804', '4775', '4772', 
    '4764', '4790', '3953', '4792', '4777', 
    '4778', '4766', '4763', '4818', '4800', 
    '4783', '4801', '4787', '4773', '4776', 
    '3745', '3719', '1966', '1938', '3709', 
    '4754', '4862', '1978', '1934', '4753', 
    '1913', '4867', '1895', '1862', '4755', 
    '1939', '2970', '1987', '1970', '2178', 
    '1910', '5137', '346', '548', '365', 
    '1575', '222', '1510', '500', '524', 
    '1856', '1859', '1870', 'Y'
  ) 
  AND lang_code = 'ru'

Query time 0.00108

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": 54,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('2998','4796','4804','4775','4772','4764','4790','3953','4792','4777','4778','4766','4763','4818','4800','4783','4801','4787','4773','4776','3745','3719','1966','1938','3709','4754','4862','1978','1934','4753','1913','4867','1895','1862','4755','1939','2970','1987','1970','2178','1910','5137','346','548','365','1575','222','1510','500','524','1856','1859','1870','Y') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}