I made it work in nvim 0.10.3 by keeping imsearch always set while toggling spelllang and keymap as desired:

func! LangEn()
    setl spelllang=en keymap=en
    return ''
endfunc

autocmd BufEnter * setl iminsert=1 imsearch=1

cnoremap <C-F1> <C-R>=LangEn()<CR>