def relpath_to_site(lang, target_lang):
    '''Get relative path from siteurl of lang to siteurl of base_lang

    the output is cached in _SITES_RELPATH_DB
    '''
    path = _SITES_RELPATH_DB.get((lang, target_lang), None)
    if path is None:
        siteurl = _SITE_DB.get(lang, _MAIN_SITEURL)
        target_siteurl = _SITE_DB.get(target_lang, _MAIN_SITEURL)
        path = posixpath.relpath(get_site_path(target_siteurl),
                                 get_site_path(siteurl))
        _SITES_RELPATH_DB[(lang, target_lang)] = path
    return path
Get relative path from siteurl of lang to siteurl of base_lang

    the output is cached in _SITES_RELPATH_DB