What is a Sapeque? Yes it's feminine in French :)
It is the most common unit of account traditionally illustrated by a copper or bronze coin with a square hole in the middle. It is used for the most common transactions and has been imitated throughout Asia. The sapeque is so emblematic of currency in China that the character qian has come to designate what we French, by a semantic shift of the same nature, call “money”.
To facilitate their transport, the sapeques are joined together in ligature of one hundred or one thousand; in the latter case the ligature is called diao and is the equivalent of a tael. The figure of one thousand sapeques per ligature is theoretical because it can most often contain less to take into account local customs and/or commissions charged by money changers.
The French expression sapèque would come from the Malay sapaku meaning - strangely compared to the interpretation that we have given - a ligature of a thousand pieces, while the English cash would come from the Sanskrit karsha (on these etymological aspects we will consult the English dictionary -Indian Hobson-Jobson). The qian transcription was previously written ts'ien in French and ch'ien in English (the apostrophe being sometimes elided for simplification).
So much for China.
Now for Indochina, I also read this interesting comment:
http://www.worldofcoins.eu/forum/index.php?topic=8746.0
The Chinese (Chữ-nôm) characters are probably more of decorative nature. The name An-Nam (安南) was also used after the colony was re-named into Indochina. The correct Vietnamese names would be: 南部 - Nam-Bộ (Cochinchine) and 東堂 - Đông Dương (Indochine).
Moreover the denomination is incomplete, the 當之二 means "with the value of 2/(something)". Since the sapèque was valued at a 500th piastre one might assume that the intended inscription was 當千之二 or "with the value of 2/1000".
So lettering of this coin is on three lines (not on two lines):
法國大 (or reverse order?)
安南
當之二
It is also interesting to note that the coin has a weight of 2 grams, and the 1 Centime coin of 1885-1894 weighed 10 grams, the weight then deteriorated.
Anyway, on the legislative article shown above, it is clearly indicated One Sapeque = 1/500 Piastre
As quoted by Joe-breuille on the french forum, Sapeque trials for Cochinchina should be corrected as well [Value 1 Sapèque (0.002)]