SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '2908', '2595', '3705', '5136', '2099', 
    '1913', '1858', '1862', '1882', '1855', 
    '1879', '5135', '1917', '2178', '2230', 
    '2076', '2338', '2945', '1910', '5134', 
    '2543', '2127', '2049', '3095', '72', 
    '1834', '21', '182', '20', '2030', 
    '1856', '1919', '1859', '1870', '137', 
    '434', '62', '11', '1702', '79', '1512', 
    '1400', '170', '1813', 'Y', '3543', 
    '10', '1520', '443', '142', '70', '345', 
    '68', '1615'
  ) 
  AND lang_code = 'ru'

Query time 0.00073

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 ('2908','2595','3705','5136','2099','1913','1858','1862','1882','1855','1879','5135','1917','2178','2230','2076','2338','2945','1910','5134','2543','2127','2049','3095','72','1834','21','182','20','2030','1856','1919','1859','1870','137','434','62','11','1702','79','1512','1400','170','1813','Y','3543','10','1520','443','142','70','345','68','1615') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}