There was a really similar post very recently. I’ll just link it here, since there was some very useful discussion included.
As for the country, your transformer should output a standardised 5 Volts over USB, regardless of your country’s voltage. It’s the current that will be different, and essentially determine how fast your gameshell charges. Older chargers would just have a lower wattage, thus would charge if slower.
You could also charge it from your computer’s USB port. Basically, if you can charge a phone with it, you can also charge a Gameshell.