How to make a per-buffer language changing command/function?

[Andrew15-5]

I've tried using setlocal spelllang=<lang>, but to change the language itself I have to (? maybe not) use <c-^> (:h CTRL-^), which is not a global thing as far as I can tell. Moreover, it only works in insert mode, so I have to make a normal mode command, that will quickly enter insert mode, press the shortcut, and exit the insert mode. It works great with 1 buffer, but when I try to change language in multiple buffers, the spelling highlighting and the inserted language are not sync.

I also added these lines:

set keymap=<other language keymap>
set iminsert=0