First, having chickens on retro doesn't break any rules. What more, I would say that seeing how good retrospective works is way more important than attending just a few of standups or even planning meeting.
Second, you should consider the impact of having chicken on retro (or any other meeting) on the team. One issue is of course the language. If everyone's English is fluent enough that you will be able to communicate everything (even if it takes more time) I wouldn't consider that a big issue. However if switching to English means someone would be effectively disabled from active participation in the meeting it may be something worth considering.
Another aspect of the same case is who will be a chicken. I was a chicken once or twice when I was a superior for the whole team and people were all stressed and it didn't work well. If you face the risk that people would censor themselves that's another issue to consider.
Third, think about the big picture. There is of course the context of the team but there is also the context of the whole organization. Even though having chickens on your retros may introduce some difficulties for the team it still can be a worthy investment company-wide. Try to judge the impact of doing such thing in wider perspective.
Finally, you can treat it as an experiment. You can just try it once or twice and if it doesn't work just don't do it anymore.
Having said all that you shouldn't enforce your ideas on team. Try to convince them, but show them the whole context.
