JWs are encouraged to dress up according to their local culture in international conventions, only for the sake of the sneaky Cult Propaganda.
That is, other JWs should see that and encourage (Brainwash) themselves as "Ahh see how the truth is spread to so many different lands. This is in accordance to the Bible that the good news would spread to all the lands. We are all speaking the same pure language. Truly this is Jehovahs Organization"
BTW, what if some tribal people - who are still naked - become JWs and attend the International conventions ?