Originally posted by tim60 That is modern Chinese, not traditional, which is what appears in Japanese.
Well, it's not totally unlike the relation between modern Greek, ancient Greek and, say, Russian, just imagine a Venn diagram of characters. Each group has unique glyphs. Some glyphs are common to only two groups. Some are shared by all three. Of course it's more complicated here, partly because of sheer number of characters involved.
Anyway, for this specific example of Thailand, in simplified (what you call modern) Chinese it's 泰国. It's 泰國 in traditional Chinese. I didn't know, so thanks mamba909 for the google link. I guess (again I don't know) that 國 isn't used in China where people use simplified Chinese, and 国 isn't used in e.g. Taiwan where people use traditional Chinese characters.
Japense never spell Thailand using Kanji characters these days, it's just spelled as タイ (pronouced "tai") using one of two phonetical character sets unique to Japanese language. Most Japanese will see 泰国 or 泰國 as a mysterious Chinese word or maybe a very old Japanese that could mean some country because they know that 国 and 國 mean country.
But the thing is that people recognize 泰, 国 and 國, as these are all included in modern Japanese character set. People know that 國 is an old version of 国, and the former is not much used except for people's names, but the old one is not completely obsoleted by the new one and it's still recognized and used and taught.