What's wrong with my localisation approach?

Dear experts and gurus,

I have this small test program and run it in ESTK with FrameMaker 14, French version:

// ES-localisation.jsx - test the localisation à la ExtendScript
var sDnav_hlpTip7   = { en: "Define the heading levels",
                        de: "Überschriften-Ebenen definieren",
                        fr: "Définir les niveaux des titres"};
wDefine = new Window ("dialog", localize (sDnav_hlpTip7), undefined);
wDefine.preferredSize.width = 200;
wDefine.show();

FM is French, as the top menu demonstrates. Nevertheless the dialogue title is English:

I follow the method described in the Javascript Tools Guide under the heading Localizing ExtendScript strings.

→ Have I overseen something?

→ Does the process look for the Windows UI language (which in my case is English), not the FM UI language - which obviously is French?