SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '4801', '4788', '4813', '4799', '4779', 
    '4776', '4820', '4780', '4784', '4797', 
    '2651', '1938', '2099', '1978', '4753', 
    '1858', '1862', '1882', '2398', '2038', 
    '1987', '1970', '2711', '3748', '5137', 
    '2030', '1942', '1856', '1870', '1859', 
    'Y'
  ) 
  AND lang_code = 'en'

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": 31,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('4801','4788','4813','4799','4779','4776','4820','4780','4784','4797','2651','1938','2099','1978','4753','1858','1862','1882','2398','2038','1987','1970','2711','3748','5137','2030','1942','1856','1870','1859','Y') and cscart_product_feature_variant_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}