I would make a distinction between a CLI (to be used by ordinary users of the program) and a scripting interface (to be used to "extend" the program, to be used

I would make a distinction between a CLI (to be used by ordinary users of the program) and a scripting interface (to be used to "extend" the program, to be used by power users and/or other programmers). And I would further submit that the same language may not be the right choice for both tasks. Tcl leans further towards being a pleasant CLI interface language, while AFAIK Lua leans further towards being a pleasant plugin development language. IMHO.