[TYPO3] Translation of frontent labels

[Michel Albert]

I am currently trying to get the frontent labels translated into french.
However I am unsuccessful so far. I even tried to manually edit the file
"locallang_core.xml" without success. What do I have to do? Do I have to
make some settings in TypoScript as well somewhere?

I tried to follow the localisation guide in the documentation files, but
that only covers the case if you want to offer several languages an let the
user chose. However, I would like to have french as default language for
everybody.

The most annoying labels currently are the "age" labels of posts
(min,hrs,days,yrs). Also, the extension "cal" is still in english.