As far as I know, it is not traditional in any sense. I recently came back from a trip to Central & East Europe and never was I served a lemon with beer, nor did I see anyone else with a lemon or orange wedge.
Anyway I too spent a some time there, actually a year, and never saw a slice or wedge or anything in a beer.
I spent quite a bit of time in Bavaria in the 80s. I never saw fresh fruit served in beer either, but something I did see that I thought was amusing was beer being sent back to the bar for not having enough head on it. Beer and wine glasses there have measured serving volume lines marked on them. In beer glasses the liquid portion of the serving must be up to that line, and foam must fill the space in the glass above the line.
And, this is really interesting, they have little metal tubes that are filled with warm water and hung in beer steins to keep beer from getting too cold when it is served in cold weather - beer warmers...