汉语里面，国家名字都是固定称谓，属于习惯用法，可能跟汉语发音习惯也有关系。在国家称谓上，没有单字，都是由两个汉字以上组成，例如“中国，美国，法国，英国，韩国”，组成由“国家简称（单字）”+“国”；但是如果翻译成中文是两个字或者以上的，比如“朝鲜，越南，巴西，埃及，荷兰，土耳其，比利时，柬埔寨，墨西哥，澳大利亚，”，后面就不用加“国”字，如果读成“朝鲜国”也没有错，但是读起来比较拗口，汉语中双音节比较多。不知道你理解了没有？:)

“国”是由古汉语演变过来的，“或”为最早的汉子甲骨文“国”，再到繁体字“國”中国古代是的国家是有城墙围起来的，里面的"或"可拆解为"戈'"一”"口",我国解放后由周恩来总理定下来，里面的玉有五德（仁、义、智、勇、洁），我们的国家、我们做人就应具备玉之五德，于是简化的“国”字就这么确定下来了。 除中国外的国家大多是音译，加于不加“国”，主要在于国家名是否是单个字，加个“国”顺口。(eg： US 美利坚合众国，简称美国, Thailand 泰国，England 英国）

对啊 汉语一般都是双音节词 加个国字是为了调节语音 这样说起来更好听 更有节奏感

同意Judy chang.汉语的一个特点是很少用单音节,单音节也要凑成双音节.比如:嘴巴=嘴/老虎=虎等^__^

i read all of presantations here made,even though to some extent what they said are reasonable ,but I think there is a core question that Chinese is a type of language deeply influenzed in the areas of rhythm, in particular when composing a poem. a fabulous essay is good not only based on using appreciate words ,but also on how to choose these words with the ideal rhythm that can express how you feel ,for example ,a pronouciation of a word sounds quiet,this kind of pronouciation can be used to express something you feel quiet,therefore ,as a saying goes"a good artical should be listened to rather than be read" in the past ,when people composed poems ,they were very strict with the rhythm. however, in modern time.how people speak is only for communication instead of seeking esthetics. in fact, good chinese poems, could be made possible by choosing adequate rhythm that can express you feelings in tandem with the meanings of these words used. even though today,the style of composing poems and essays has changed a lot,yet they are sitll deeply infulensed by the rhythm. i think that this is a distinctive feature of chinese .let's come back to your question.as a matter of fact, why not put "国" before ‘澳大利亚"or"俄罗斯",just put it before "中""美",in my view,is mostly beacause put "国'before "美" just one singlar sounds better.and not put "国”before"澳大利亚" is because when people translated "Austrilia" into chinese he had considerated the influence of rhythem on the chinese character translated into ,that is "澳大利亚" sounds good enough. i belive that you have found that there are so many phrases consisting of two characters,in fact,that is exactly a reflect of the use of rhythm because they sound good rather than strange. however, most of the time,we use the words according to what they have now been instead of create on your own according to the grammer you have got. all above is what i know.i hope they can help you a little.