Thanks David for your quick and precise answer.
I updated the documentation page to:
==Moodle placeholders must not be translated==
The following words, when enclosed within {curly brackets}, are placeholders for Moodle names of variables, and they should not be translated, but must remain as they are within the curly brackets:
firstname, lastname, username, email, city, country, lang, timezone, mailformat, maildisplay, maildigest, htmleditor, ajax, autosubscribe , institution, department, idnumber, skype , msn, aim, yahoo, icq, phone1, phone2, address, url, description, descriptionformat, password, auth, oldusername , deleted, suspended, course1, course2, course3, course4
They must also be written exactly like this in all translations of the Documentation pages (such as this page); but the documentation writer/translator might like to include a translation enclosed in brackets. For example in the spanish Documentation pages you might find: password (contraseña).
The fact is, no placeholders can be translated in AMOS. There is no official list of "reserved" $a properties. Whatever xyz is put in {$a->xyz}, it must be kept as it is in AMOS.
I hope this is less confusing
I would greatly appreciatte if either you, Koen, Helen Foster, or somebody else with a better English grammar than mine, would improve the http://docs.moodle.org/26/en/Translation page
By the way, is the fj language pack, which is quite different from the rest of the language packs. wrong ? Maybe you or Koen could write the fj language pack maintainer about this.
Thanks again.