SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1449', '1201', '1219', '1269', '5900', 
    '5904', '5907', '1280', '1254', '5909', 
    '1203', '1199', '1241', '5915', '5916', 
    '1204'
  ) 
  AND lang_code = 'ru'

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 16,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1449','1201','1219','1269','5900','5904','5907','1280','1254','5909','1203','1199','1241','5915','5916','1204') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}