The current state can be found at [http://www.tcl.tk/doc/howto/i18n.html#APIs].

The [msgcat] package provides internationalization on the script level.
This functionality is missing on the C function side.
May be something like the following idea could be incorporated in the core.

[msgcat] uses dictionaries to save the translation strings.
Access to nested dictionaries is not (yet?) exposed.
So the msgcat::ns function saves the available translation strings in an array.
The function should be called after setting the locale (can be done with a trace on msgcat::Loclist)
and after setting the translation strings.
======
proc ::msgcat::ns {ns src} {
  variable Msgs
  variable Loclist

  if {[string range $ns 0 1] ne {::}} {set ns ::$ns}
  namespace eval $ns {}
  catch {unset $ns}
  array set $ns {}
  foreach loc $Loclist {
    if {[dict exists $Msgs $loc $ns]} {
      dict for {key value} [dict get $Msgs $loc $ns] {
        if {![info exists $ns($key)]} {set $ns($key) $value}
      }
    }
  }
}
======

C-function to get a translated string.
======
const char *Tcl_Msg(Tcl_Interp *interp, Tcl_Obj *ns; Tcl_Obj *text) {
  Tcl_Obj *myVar;

  myVar = Tcl_ObjGetVar2(interp, ns, text, TCL_GLOBAL_ONLY);
  if (myVar == NULL) {
    return (Tcl_GetString(text));
  } else {
    return (Tcl_GetString(myVar));
  }
}
======

C-Extension (p.e. 'Ext1' in namespace 'ext1')
======
Tcl_Interp *myInterp; /* used interpreter */
Tcl_Obj *myNs; /* used namespace, also name of array variable to hold translation strings *
#define EXT1_MSG(text) Tcl_Msg(myInterp,myNs,Tcl_SetStringObj(myText,text,-1))
int Ext1_Init(Tcl_Interp *interp) {
  ...
  myInterp = interp;
  myNs = Tcl_GetStringObj("::ext1",-1);
}
void Ext1_command() {
  printf(EXT1_MSG("some dummy text at line %d",__LINE__);
}
======

Find all translation strings of extension 'Ext1' with:
======
proc findstrings {function cfiles} {
  set regex $function
  append regex {[ \t]*\([ \t]*("(?:\\"|[^"])[ \t]*)}
  foreach f $cfiles {
    set fd [open $f r]
    set c [read $fd]
    close $fd
    foreach {x text} [regexp -all -inline $regex $c] {puts $text}
  }
}
findstrings EXT1_MSG [glob *.c]
======

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