change the permalinks before plugin-installation

I want to change my permalinks before plugin-installation, but I am not sure what you mean by this phrase of your documentation:

“When using the non default permalinks, pages (and posts as well if your permalink structure is /%postname%/), must not have a language code as permalink.”

I have currently the default permalinks configured and wanted to change to /%postname%/. My website shall be like:

domain.de/de/postname
domain.de/en/postname

According to your documentation I should not choose this kind of permalinks. But I have this permalink structure on a test installation, using with polylang since a few weeks, with no error.

What are the unexpected results that might occur? Or did I misunderstand something? (That is not my native language)

http://wordpress.org/plugins/polylang/