What "polite" questions can be asked to a foreigner?

Normaly and practically it is other way round it is Non Indians who starts the conversation first with Natives. First thing they say is warm Hello and introduce themselves and then conversation may lead to answering his basic questions or query, further it is like normal conversation between anybody.

This is India, the land of maya and the chakravyuha and the sleight of hand.

Everything is real

And everything has ISI mark.

I think that they (foreigners) are smart enough some times they get a hint about the attitude of the person they are talking to. So we must be original in what we talk

I have not come across any Indian who would object to any question. Ask, they are waiting for it.

Can I date your sister.?

You just did

ah, yes of course!

No, that is not culturally appreciated. My sister will go with the family or her girl-friends. When the time comes, we will marry her to a suitable person after getting her approval. The bride-groom should be from our caste. (Unfortunately, I have no 'real' sister, cousins of course. My daughter was the first girl born after three generations)

Must have been Mothers among them!

The opener for conversation with an Englishman: "How was the weather at home?"

So that is why her brother got so weird..

Well, recognize the cultural difference. Nothing weird in it. Do not get it. Some people thought my grandfather had a mantra for birth of sons in the family (he was a Sanskrit scholar).

Interesting response. I had intended my comment as a joke since you assumed I was ignorant of Indian culture (all forens must be). As usual it fell flat and I apologize for moving you out of your comfort zone. But, the response reminded me of my late mother in law's comments on unresolved gender issues in India. She was a very sharp lady & dearly lamented..

Salary is a taboo argument here in Italy: people don't like to discover to friends what they earn so avoid direct question, especially if they are in a group!

But you can turn question in another way:
Is life in your country very expensive?
What is a medium salary in your country?
Is buying/renting house expensive in your country?

It could be they answer on general basis or telling "My salary is...."
Probably they are interested to know the same about your country/your salary to make comparison.

Marriage is not a taboo argument but only if it is a happy marriage

Origin country is a polite question....the only thing is you could be the millionth person to ask this on the same day

Italian people are always happy to talk about food
Other arguments:
- weather / climate in your country?
- typical sports in your country?
- Holidays in your country? (how many days per year? In which period? In which places?)
- yoga - Do you practise it in your country? (main part of italian people believes all Indians are silent people who stay all the day in lotus position, closed eyes and Samadhi status )

I thought it was a good joke, I just wish I had said it!