TITLE: Why do the French count so strangely?
QUESTION [63 upvotes]: Today I've heard a talk about division rules. The lecturer stated that base 12 has a lot of division rules and was therefore commonly used in trade.
English and German name their numbers like they count (with 11 and 12 as exception), but not French:
  # |   English |           German | French
-----------------------------------------------
  0 |      zero |            null  | zero
  1 |       one |            eins  | un
  2 |       two |            zwei  | deux
  3 |     three |            drei  | trois
  4 |      four |            vier  | quatre
  5 |      five |            fünf  | cinq
  6 |       six |           sechs  | six
  7 |     seven |          sieben  | sept
  8 |     eight |            acht  | huit
  9 |      nine |            neun  | neuf
 10 |       ten |            zehn  | dix
 11 |    eleven |             elf  | onze
 12 |    twelve |           zwölf  | douze
 13 | thir|teen |       drei|zehn  | treize
 14 | four|teen |       vier|zehn  | quatorze
 15 |  fif|teen |       fünf|zehn  | quinze
 16 |  six|teen |       sech|zehn  | seize
 17 |seven|teen |       sieb|zehn  | dix-sept
            18 and 19 are "regular"
 20 |    twenty |          zwanzig | vingt
 21 |twenty-one |  ein|und|zwanzig | vingt et un
 22 |twenty-two | zwei|und|zwanzig | vingt-deux
            23 - 69 are "regular"
 70 |  seven|ty |         sieb|zig | soixante-dix = 60 + 10
              ....
 80 |   eigh|ty |         acht|zig | quatre-vingts = 4*20 ?!?!
 81 |eighty-one |  ein|und|achtzig | quatre-vingt-un = 4*20 + 1
      ...

So my question is:
Why do French count so strangely after 79?
(Are there other languages that count similar? What's the historic / mathematical reason for this system?)
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REPLY [2 votes]: As former Chinese then French colonists we Vietnamese dropped the Chinese script and took on the Roman alphabets and the Arabic numbers as our own, thank to France. But we didn't take on the French counting system. Ours is very straight forward, and like Chinese, is mostly a monosyllabic language. 
một (one),
hai (two),
ba (three),
Bốn (four),
năm (five),
sáu (six),
bảy (seven),
tám (eight),
chín (nine),
mười (ten),
mười một (ten one = eleven),
mười hai (ten two = twelve),
hai mươi (two tens = 2(10) = 20),
hai mươi một (two tens one = 2(10) + 1 = 21),
ba mươi (three tens = 3(10) = 30),
ba mươi một (three tens one = 3(10) + 1 = 31),
một trăm (one hundred),
một trăm một (one hundred one = 1(100) + 1 = 101),
hai trăm (two hundreds).