Eleven and Twelve

What is it about these 2 numbers? Why are they not teens?

As in 12 should be twoteen and 11 oneteen.

Another example is the german: Zwolf, elf; Zweizehn, einzehn

I believe this is true for most indo-european languages (apart from italian strangely).

I suspected they would be remnants of a duodecimal counting system, but apparently not. Here's a quote from Wikipedia for you:

Germanic languages have special words for 11 and 12, such as eleven and twelve in English, which are often misinterpreted as vestiges of a duodecimal system. However, they are considered to come from Proto-Germanic *ainlif and *twalif (respectively one left and two left), both of which were decimal.

http://en.wikipedia.org/wiki/Duodecimal#Origin

It doesn't explain French's onze and douze, etc. though.