Locale shouldn't be based on selected language only

This appears to be feature of all Gnome systems but it's wrong. System locale is always changed according to selected language, but what if user wants to use English language and his national locale settings for date, time, papersize...? It doesn't look nice having american date style in ie Thunderbird whether I rather see german datestyle.

There is a way to accomplish this by creating empty file /etc/X11/Xsession.d/00_lc_all
and put the following content inside (example for slovenian locale)

unset LC_ALL
export LC_CTYPE=sl_SI.UTF-8
export LC_NUMERIC=sl_SI.UTF-8
export LC_TIME=sl_SI.UTF-8
export LC_COLLATE=sl_SI.UTF-8
export LC_MONETARY=sl_SI.UTF-8
export LC_PAPER=sl_SI.UTF-8
export LC_NAME=sl_SI.UTF-8
export LC_ADDRESS=sl_SI.UTF-8
export LC_TELEPHONE=sl_SI.UTF-8
export LC_MEASUREMENT=sl_SI.UTF-8
export LC_IDENTIFICATION=sl_SI.UTF-8
export LC_MESSAGES=en_US.UTF-8

After rebooting the system, we are able to use English language with our national locale settings in almost all applications.

KDE does have those specific locale settings. Could it be possible to get something like this by using Gnome GUI?