Essentially there are an unlimited number of fields that could be substituted into templates. If you want to contribute good code to add such a field its likely to be accepted (at least until the dialog is full).
But in the meantime you can edit the template files to set the language you want, just copy the template file to your `config directory/templates` from the systems `share/geany.templates` and edit. You can even have several versions of the same template so long as the names are different, maybe `file.en.html` and `file.fr.html`.