It sounds a bit odd to me when used with countries. But it is not necessarily wrong.
A guess, but maybe it sounds odd because "beside" is used with things that can be moved or can move; "next to" tends to be used for geographical features. In this case however, China is not next to Japan - even "near" might be a bit of a stretcher.
Actually there is a distance of about 750 miles between China and Japan - stretching the meaning of "beside" a bit - and I don't think that we normally use "beside" when talking about the location of countries - maybe "next to" would be better, but untrue in this case.
I have been told that when you talk about a country it is always followed by "orszàg" : Japanorszàg Kinaorszàg mellett van isn't it correct ? other meaning ? Thanks
not always, these two are exceptional (among many others).
No logic there though, you need to memorize... sometimes the "ország" is added (like in svéd/Svédország (swede/Sweden)), sometimes the "i" suffix indicates that someone arrives from that country (koreai/Korea (korean/Korea), sometimes the "ia" suffix is used for the country (portugál/Portugália (portuguese/Portugal), and sometimes it's just weird and irregular (japán/Japán (japanese/Japan)