1. Language
Never make fun of others' language including words and accent. It is their mother tongue and nobody will like it if you are insulting their mother.
2. Food
Do not give negative comments on the food they are eating. They are not going to change their eating habits or the food they have been eating since childhood.
3. Religion
Respect their religion and let them follow it. Do not comment on it as it will hurt their feelings.
4. Comparison
Last but not the least, never compare your financial status, education, health, children, successes, wisdom and situations with them. It will do no good.
Your suggestions are noted well @Somesh
I agree with you on all the four points @Somesh
Wonderful post @Somesh. All the four points are true and it is the root cause of conflicts. Hope to see a prosperous world with this.