Hi Luc!
> I'm writting a paper about a software I develop using Tcl/Tk. As it is the first time I need to cite Tcl/Tk, I'm wondering which reference should be used to cite Tcl/Tk ?
I'd say it depends what the primary purpose of the reference is?
Is it about pointing the reader to Tcl/Tk as a language as such?
-> In whatever bibliography style used, make it an "electronical resource": URL
http://tcl.tk/ plus last accessed date?
Is it about pointing the reader to some language reference, introduction (because this is required or expected by the reader)?
-> Use Ken's or Ashok's recent reference books:
Tcl and the Tk toolkit (8.5 + Tk)
The Tcl Programming Language: A Comprehensive Guide (8.6 no Tk)
Is it about a particular language feature, which is key to understanding your tool?
--> Then find sth. more specific (a TIP as an electronic resource, or particular fossil commit?). There are different accepted ways to point to individual code repositories, commits or tags/releases as an "electronical resource" (URL plus repo identifiers) or even DOIs, e.g.:
https://guides.github.com/activities/citable-code/
Stefan