English used to do this too. The most famous example is the first line of Lincoln’s Gettysburg address:
Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
Joke aside, it's not taught as 4 × 20 +10 but simply “90 is pronounced quatre-vingt-dix” — which kinda is a mouthful, but you rarely count to 90 as a kid anyway.
As guy who hate French language and was learning in 1999 I can confirm it was pain to read the topic of lesson and the date. I was so happy when we switched to 2000.
Whole generations of French students that have no idea they escaped having to write "mille neuf cent quatre-vingt dix-neuf" over and over again, in cursive of course.
Nineteen is dix-neuf though. Which is literally ten-nine. 11-16 all have an equivalent word to the English “teens.” Quatorze for example instead of dix-quatre for 14.
The British one has the "color" changed changed to "colour" due to British spelling of color.
The Spanish one has an upside down semi colon because in Spanish you write questions like this: ¿Is this an example question?
The French one is because the French number system makes absolutely no sense and to say 99 you have to say quatre-vingt-dix-neuf (meaning 4 x 20 + 19).
The Danish word for 99 is nioghalvfems, which literally means "nine and half five." Which you could be forgiven for assuming meant 11½. The trick is that a) "half five" actually means 4½, as in half less than five, and b) it's implied that you're supposed to multiply the second part by 20. So the proper math is 9 + (-½ + 5) * 20 = 99.
Wait, spanish doesn't do the "we don't have a word for that number, just do math instead" counting system?? I thought the romance languages were tight!
Well, there isn't a word for 99 in Spanish or English, in both languages we say 90+9, so that counts as maths.
If you are asking about words for 70, 80 and 90, that is a peculiarity of French, and not even all dialects, some dialects have septante, huitante/octante and nonante for those.
French is the only Romance language that does the count-by-twenties thing, as far as I know, but apparently some of the Celtic languages do it too. So French may have picked it up from Breton (or Gaulish or who-knows).