So here it goes like this:

First: You should lookup for your wanted language-layout option name in output of the command below.

localectl list-x11-keymap-layouts

After that: place following code on your i3 configuration.

exec_always "setxkbmap -model pc104 -layout us,ir -variant ,, -option grp:alt_shift_toggle"

replace ir with yours. ir is a layout name for Persian language ( my native language ).

After that, reload the i3 configuration and thats should work.

Lets break that command above :

exec_always : In the context of i3wm, this command basically execute the command that you write in front of it ,always when i3wm initialized.

setxkbmap : This command in Linux is used to set the keyboard layout and options for the X Window System ( X.org is the open-source implementation of the X Window System and i3 is based on xorg )

-model pc104 : This specifies the keyboard model. The pc104 is actually what we call a full size keyboard ( with numpad and etc, 104 keys in total )

-layout us,ir : This option specifies the keyboard layouts to be used.

-variant ,, : This specifies the variants of the specified layouts. we leave it empty so it uses the default variant.

-option grp:alt_shift_toggle : This sets options for switching between the specified keyboard layouts. I think the command itself is self-explanatory.