That can be done by adding the specific keyboard layout command into a .desktop file saved in ~/.local/share/applications along the other desktop files that launchers like Synapse can therefore access.
The keyboard layout command to select a certain layout is like:
setxkbmap us intl
Change us with the desired language and intl with variant if necessary.
For French it should be setxkbmap fr. Etc. A list of all possible keyboard layouts and other parameters can be found in: /usr/share/X11/xkb/rules/xorg.lst.
The desktop file should have a form similar to
[Desktop Entry]
Name=US - Keyboard US International dead Keys
Exec=setxkbmap us intl
Icon=path/to/some/icon/file
Terminal=false
Type=Application
Categories=Application;Utility;
