Nintendo has had a profanity filter in place for their systems for a long while now, even going so far as to drop Swapnote to ensure that kids are safe. Of course, someone went ahead and datamined the 3DS and found a full list of banned words, which you can find here.
They range from the obvious to the obscure (“analintruder”) to the bizarre (“chinaliberal”) to the religious (“jesussucks”). Nintendo thought of everything!
Full list of censored words on 3ds [reddit]