[dsr_]

English: one two three four five six seven eight nine ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen twenty: normal from here on (twenty-five, seventy-three, one million forty-eight thousand five hundred seventy-six)

Spanish: uno dos tres.... (15) quince, normal from 16 on.

French: normal begins at 17 (dix-sept), but runs into trouble later on (courtesy of a friend):

70 - soixante-dix, sixty-ten, 80 - quatre-vingts, four-twenties ("score"), and 90 - quatre-vingt-dix, four-twenty-ten.

That makes 99 "quatre-vingt-dix-neuf".

German is the same pattern as English, normal after 20, but prefers the smaller numbers first.

I have heard that the Russians need to study their conventions intently...

[andrewla]

The other interesting thing about French is that Swiss French and Belgian French have both fixed this - ninety is “nonante” instead of “four twenty ten”. Belgians use “quatrevingt” instead of the Swiss French “huitante”, though.

The oddest thing of all is apparently the simplified form is an old French usage that got replaced by the Gettysburg-Address-style “four score” more recently. Why they would change in that direction is a mystery to me.

[chr1]

Russian numbers are ten based and rather straightforward;

odin dva tri ... desyat (10) odinnadtsat dvenadtsat trinadtsat ... dvadtsat (20)

there is no irregularity other than 40 which is sorok instead of chetiredesyat

[Mediterraneo10]

Foreign learners do not generally find Russian numbers to be "straightforward". Not only do nouns qualified by numbers take different numerative cases (the genitive singular for 2–4, the genitive plural for 5–10), but the declension of compound numerals is also surprising for speakers of many languages.

Compound hundreds are also unpredictable. Yes, historically the varying forms are clear (the back yer in Common Slavonic sŭto ‘100’ was lost or strengthened depending on position), but learners today without any background in the Slavic languages simply need to learn them by rote.

[chr1]

True, sound changes like dvesti÷trista÷semsot make learning harder, but i'd say that's not really the property of the number system but of a language as a whole, since similar shifts are present in other places too.

For me Russian numbers were simpler than French because you could learn the basic idea quickly, and then pick up subtle sound shifts by watching tv/reading.

(disclaimer, i am not a native speaker but learned Russian at school age so may be misgudging the difficulty of the language)