Forum OpenACS Q&A: AOLserver/openACS and other scripting languages (besides Tcl)
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.
Whether the programmers you hire already know Tcl or not is close to the last thing you should be worrying about. Any good programmer can and will reach basic proficiency with Tcl in a few days, and true expertise in a few weeks (or perhaps months). This is not hypothetical, I've seen that myself first hand.
Actually, I've also seen people I wouldn't be willing to hire as programmers, because they weren't really good enough, also learn enough Tcl to do basic OpenACS work within days. Tcl simply isn't that hard.
And Dave is exactly right that when it comes to learning OpenACS, Tcl is the quick and easy part.
Many developers are just lazy to learn new stuff besides of the scripting language they already know, that'll be very first trouble you'll get.