It’s interesting how different languages count from 11 to 20.

Why Is It ‘Eleven, Twelve’ Instead of ‘Oneteen, Twoteen’? • 2016 Feb 24 • Arika Okrent • Mental Floss

Latin does have something akin to oneteen, twoteen, threeteen, fourteen, fifteen, sixteen, seventeen

11

ūndecim = ūnus decem (one ten)

12

duodēcim = duo decem (two ten)

13

trēdecim = trēs decem (three ten)

14

quattuordecim = quattuor decem (four ten)

15

quīndecim = quīnque decem (five ten)

16

sēdecim = sex decem (six ten)

17

septendecim = septem decem (seven ten)

Spanish basically just transforms the Latin words for 11 to 15:

11

once < ūndecim

12

doce < duodēcim

13

trece < trēdecim

14

catorce < quattuordecim

15

quince < quīndecim

After 15, Spanish actually uses the convention for numbers > 20.

16

dieciséis = diez y seis (ten and six)

17

diecisiete = diez y siete (ten and seven)

18

dieciocho = diez y ocho (ten and eight)

19

diecinueve = diez y nueve (ten and nine)

compare to:

21

veintiuno = veinte y uno (twenty and one)

22

veintidós = veinte dos (twenty two)

23

veintitrés = veinte tres (twenty three)

etc.

For 18 and 19, though, Latin uses subtraction:

18

duodēvīgintī = duo de vīgintī (two from twenty)

19

ūndēvīgintī = ūnus de vīgintī (one from twenty)

Which might be related to how Roman numerals are written:

18

XIIX (usually this is now written as XVIII with the alternative form octōdecim but XIIX is definitely attested to)

19

XIX

Compare this to a completely unrelated language: Tagalog

11

labing-isa = labi na isa (surplus of one)

12

labing-dalawa = labi na dalawa (surplus of two)

13

labing-tatlo = labi na tatlo (surplus of three)

etc.