Which of 法国, 法兰西, and 法兰斯 is the most common word for France?
For your question, as a native Chinese speaker, I use 法國。

(For illustration purposes, I have below pronunciations in Mandarin for your references. P: Pinyin, I: IPA)



There seems to be some categories of way we name countries:


Sound and meaning transcription.


e.g. 新西蘭 (New Zealand)


新 means new
西蘭 is the sound transcription

美利堅合衆國 (USA)


美利堅 is the sound transcription of America
合衆國 corresponds to the meaning of "United States".


Sound transcription


e.g. 紐西蘭 (New Zealand)
意大利 (Italy)
澳大利亞 (Australia)
俄羅斯 (Russia), but the sound transcription may have come from the Mongolian transcription, but there isn't concrete proof, since similar transcriptions may have existed > 650 years.

Shortened transcription (1 character + 國)


e.g. 美國 (USA)



Sometimes, we may name some countries with more than one way using rules above.

法國、俄國、英國、美國. These short forms are the most commonly used names for the countries: France, Russia, UK, USA. This may only work if the country's name can be represented with only 1 Chinese character (since Chinese love disyllabic words more than monosyllabic words). Otherwise, we would use rules below.

From personal experience, we don't usually say the full form of names if a short form is commonly used, perhaps except for 俄羅斯 (Russia).

Here are some we don't extract the first character + 國. For example, 意大利 (Italy), 新加坡/星加坡 (Singapore). Singapore may be called 星洲/星島 (Singapore Island). 澳大利亞 may be shortened to 澳洲 (Australia Island). If the sound transcription of the name is already 2 syllables, then we never shorten it.



I have no conclusive answer for the reason of the pattern though.



Pronunciations:


新西蘭 (P: xin1xi1lan2 I: /ɕɪn55.ɕi55.lan35/)
美利堅 (P: mei3li4jian1 I: /meɪ21.li51.tɕjɛn55/)
合衆國 (P: he2zhong4guo2 I: /xɤ35.ʈʂʊŋ51.kuɔ35/)
紐西蘭 (P: niu3xi1lan2 I: /njoʊ21.ɕi55.lan35/)
意大利 (P: yi4da4li4 I: /i51.ta51.li51/)
澳大利亞 (P: ao4da4li4ya4 I: /ɑʊ51.ta51.li51.ja51/)
俄羅斯 (P: e2luo2si1 I: /ɤ35.luɔ35.sɯ55/)
星加坡 (P: xing1jia1po1 I: /ɕɪŋ55.tɕja55.pʰɔ55/)
澳洲 (P: ao4zhou1 I: /ɑʊ51.ʈʂoʊ55/)