vim:keys: Cycle spelllangs only between en & de

If :h 'spelllang' is something else, do nothing.

A possible future improvement could be to keep a list of the languages
to cycle between and first adding any other languages to this list so
that one could return to the original setting. I see this especially
useful when the current setting is `en,de`.

.config/vim/vimrc.d/40-keys.vim

@@ -60,7 +60,7 @@ noremap <leader>[S [S1z=<C-O>
 function! CycleSpellLang()
 	if (&spelllang == 'en')
 		setl spelllang=de
-	else
+	elseif (&spelllang == 'de')
 		setl spelllang=en
 	endif
 endfunction