Arguable solution to generate language switch links using lang (2 letters code) injection in post-epilogue typeswitch function site:localize: the purpose of the site:lang function is to generate a relative URL when an absolute URL (starting with '/') would be rewritten in site:localize.

language selection menu

declare function site:lang( $cmd as element(), $view as element() ) as element()*
{
  let $lang := $cmd/@lang
  let $qs := request:get-query-string()
  let $action := if ($cmd/@action = ('GET', 'POST')) then () else concat('/', $cmd/@action)
  let $slash := if ($cmd/@type = 'item') then () else '/'
  return
    (
    if ($lang = 'en') then
      <span id="curLg">EN</span>
    else
      (: generates a relative path to avoid href rewriting in site:localize :)
      let $xtra := if (ends-with(request:get-uri(),'/')) then '../' else ()
      let $path := concat(substring($cmd/@base-url,2), $cmd/@trail, $action)
      let $rewrite := concat($xtra, replace($path, "[^\\/]+(\\/)?",'..$1'),'/',$path)
      return
        <a href="{$rewrite}{if ($action) then () else $slash}{if ($qs) then concat('?', $qs) else ()}" title="English">EN</a>,
    <span> | </span>,
    if ($lang = 'fr') then
      <span id="curLg">FR</span>
    else
      (: hard wire 'fr/' because it will not be rewritten in site:localize since @def-lang = @lang :)
      <a href="{$cmd/@base-url}fr/{$cmd/@trail}{if ($action) then $action else $slash}{if ($qs) then concat('?', $qs) else ()}" title="Français">FR</a>
    )
};