Maybe you just do not try hard enough? No, seriously - that is a known problem reported as MDLSITE-1359. I just did not get to look at it closely so far. I just did some debugging now. Having your langpack fully translated helped me a lot.
Firstly, there was a bug in AMOS that it did not consider tool_assignmentupgrade as a standard plugin in 2.3 while it should be. This was fixed, so some missing strings can appear in stats at the download page again.
Then, there are two malicious strings reported by my script:
langconfig.php does not contain parentlanguage
tool_assignmentupgrade.php does not contain notsupported
Both these strings are declared as empty in the English pack. So it is perfectly OK to leave them untranslated in French (and the Translator even does not let you to commit an empty string which is yet another known issue). So these two strings are not in fr.zip. I thought I found the reason of the problem - it wrongly handles strings that are empty in English and missing in French. Simply said, such strings should be considered as translated. BUT. What worries me is that the stats display 17845/17846 French strings translated. If I was right, there should be two strings reported as missing, not one...
OK, let me work on this yet. I'll see what can be done. Thanks for the reminder.
