SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1934', '1862', '1855', '1939', '1874', 
    '3101', '3102', '3087', '3090', '3093', 
    '3094', '1856', '1859', '1870', 'Y', 
    '229', '231', '723', '511', '658'
  ) 
  AND lang_code = 'ru'

Query time 0.00046

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": 20,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1934','1862','1855','1939','1874','3101','3102','3087','3090','3093','3094','1856','1859','1870','Y','229','231','723','511','658') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}