Javascript conflict? Is the theme using ajax on frontend?
I am currently working on a way to auto translate the categories. That is: if your theme is querying a specific category, you just have to translate it and Polylang will automatically sends the correct category to the theme. Maybe you can try this.
Paste http://pastebin.com/utLLn1cu in a php file that you upload in your plugins directory, then activate the new plugin.