SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '91', '136', '89', '55', '52', '58', 
    '39', '56', '53', '44', '45', '54'
  ) 
  AND lang_code = 'ru'

Query time 0.00258

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": 12,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('91','136','89','55','52','58','39','56','53','44','45','54') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}