The language you choose is a little importance. Tcl itself is very small.
http://tcl.tk/man/tcl8.4/TclCmd/contents.htm
Tcl has around 88 commands.
The OpenACS core has 2187 procs.
All OpenACS packages have 7553 procs.
So you can see that learning Tcl is a 1 day project, while learning OpenACS taks a lot more. The APIs of the libraries you use are what you need to learn.