I use two keyboard layouts en_US and ru_RU
My 
# locale
 LANG=ru_RU.UTF-8
 LC_CTYPE="ru_RU.UTF-8"
 LC_NUMERIC="ru_RU.UTF-8"
 LC_TIME="ru_RU.UTF-8"
 LC_COLLATE="ru_RU.UTF-8"
 LC_MONETARY="ru_RU.UTF-8"
 LC_MESSAGES="ru_RU.UTF-8"
 LC_PAPER="ru_RU.UTF-8"
 LC_NAME="ru_RU.UTF-8"
 LC_ADDRESS="ru_RU.UTF-8"
 LC_TELEPHONE="ru_RU.UTF-8"
 LC_MEASUREMENT="ru_RU.UTF-8"
 LC_IDENTIFICATION="ru_RU.UTF-8"
 LC_ALL=
# gtk3 = 3.18.6
# gtk2 = 2.24.29
If I press the "dotkey" on numpad in gnome-terminal in both en and ru layouts of keyboard it will be print comma. If I press ctrl+"dotkey" it will be print dot in en layout and comma in ru layout.
In a widget factory in gtk2 mode "dotkey" prints dot in en layout and comma in ru layout , but in gtk3 mode it prints comma in both layouts. So in many gtk3 applications like nautilus, fileroller, remmina, etc. comma has printed instead of dot in en layout. Ctrl+"dotkey" works only in gnome-terminal.
If I set LC_NUMERIC="en_US.UTF-8" all gtk3 applications will print dot in en layout and comma in ru layout as expected.