[#413] initialization of preview generator now uses the global tracim language variable

tracim/tracim/templates/file/getone.mak

               (function initialisePreviewGenerator() {
                 previewGenerator(
                   document.getElementById('previewGenerator'),
-                  'fr_FR',
+                  globalTracimLang,
                   [
                     % for one_url in url:
                       '${one_url}',