Localization problem with LANG=fr_FR

Perhaps I did some errors but I have some localization
problems. As I am french and use linux in non utf8 mode,
I have set LANG=fr_FR in lang.sh and in plasma-localerc.

With that I have a french keyboard where dead keys are
really dead except in xterm.

To enter circumflex and diaeresis characters with AltGr,
I have to install keyboard with the variant "Francais
obsolete variante", all other variants dont work.
It is not ideal, but it works.

As I see in lang.sh
# Non-UTF-8 options for en_US:
# export LANG=en_US.ISO8859-1
I have changed LANG=fr_FR to LANG=fr_FR.ISO8859-15
Then I have a french keyboard with dead-keys.
The keyboard is also OK if I install the keyboard
without any variant.

Perhaps I am wrong, but it seems a KDE bug with
LANG=fr_FR and also other languages ?

With ISO8859-15 a lot of applications are in english
instead of french. For instance konsole is in english
and language parameter in konsole does nothing.
But LANG=xx_XX.ISO8859-x is not the Slackware standards.