[tldoc] translations for TL21

[Karl Berry]

Hello TeX Live translators and all.

First, if you handled the translation last year, please tell me if
you'll be able to do it again this year. If not, no problem of course,
I'd just like to know so we can try to find other volunteers.

When we start the pretest for TL21, we're thinking that, for the first
time, we will also enable translations for messages in tlmgr and
install-tl. We'll start with just a few messages, that also show up in
the GUI front-ends, to make sure there aren't any unexpected problems.

Question: is the current simple procedure for handling translation
updates sufficient? That is, you pull the messages.pot file from svn or
rsync or the web, and then email new translations to this list or commit
yourself.

This is fine as far as I'm concerned, but if there's a desire to have a
fancier mechanism, presumably web-based, we could discuss it. However: I
don't much care for weblate and its constant pushing of its paid
service, and the original translationproject.org might be too
rigorous/Unixy of a process. I haven't looked into alternatives.

Anyway, I hope to have the main TL manual ready for translation updates
by early March or so. --thanks, karl.