Problem with dead-key in linux (debian + KDE)

I am trying to have a clean configuration for my keyboards and layout. I want to be able to switch between FR layout and US international layout with dead keys. When I switch using the GUI it works well, except that the dead keys doesn't working within QT applications. In other words, it works for Firefox and other non QT apps, but not for the majority of the programs I use since I am with KDE. Thanks to [this link](https://bugreports.qt.io/browse/QTBUG-25360) I realized that when in a qt terminal, $LANG is always set to ia_FR.UTF-8. If I manually change that (put it in my .bashrc) or by running application prefixing them like this:LANG=en_US.UTF-8 kmail it works. Then, where LANG is defined by the OS? why QT apps and GTK apps seems to not behave the same?