Here is an article which explains why there are “eleven/twelve” instead of “oneteen/twoteen”. Well, in my mind the answer should be much more simpler: because the ones who invented the words “eleven/twelve” were using the “duodecimal“ number system, instead of the “decimal” number system like us today. So for them, “11/12” are concepts as “1/2”, not concepts deriving from them. So they had their own names “eleven/twelve” as “one/two”, instead of “oneteen/twoteen”.
Some more evidences of my guessing:
A “dozen“ equals to 12.
A “gross“ equals to 12 dozens(144).
A “great gross (or grand gross)“ equals to 12 grosses(1728).
A foot equals to 12 inches.
Is it obvious?
Correspondingly, in the Chinese language, since the ones who invented the names of “11/12” were using the “decimal” number system just like us today, for them “11/12” were concepts deriving from “1/2”. For “1/2” were named “一/二”, they named “11/12” as “十一/十二”. As you may already realize, yes, in Chinese “十” means “10”.