Polylang one page for more languages

Hello,

I have create a website in two languages, English and German, with polylang. Now most pages are in English and German but a few like the Gallery or Impressum should only be in one language but accesible from both languages without switching the menus.
I checked the Polylang Documentation and found this:
http://polylang.wordpress.com/documentation/documentation-for-developers/tips-and-tricks/how-to-display-the-default-language-post-if-the-translation-does-not-exists/

Unfortunately I have no clue where to add this so it would work. It would be great if I could get some help on this.
Thanks.