If we are travelling to a non-Muslim country like Bangkok, how should we proceed with the food?

If the restaurant serves pork and it's hard to find those who do not, can we eat there but keep to fish, rice and vegetables?

It is not allowed for a Muslim to sit at a table where alcohol (and by extension any other haram stuff) is circulated and eaten because, what has been narrated by Ahmad from Ibn Umar and at-Tirmidi from Jabir that the Prophet (ﷺ) said, “Whoever believes in Allah and the Last Day should not sit on a table where alcohol is circulated” [1]. Therefore, it is not permissible for a Muslim to eat in the restaurants where alcohol, pork and haram substances are consumed and sold. There are Muslims in Thailand and one hopes that there are halal restaurants in Bangkok. You could do some research about their locations before travelling.

Having said that, Islam web (Fatwa website of Qatar's Ministry of Endowments and Islamic Affairs) says that in cases of necessity, you can eat in such restaurants but you should not sit at a table where haram things are served or circulated. If possible you must go in the restaurants at times when they do not serve haram food and drink.

And Allah knows best.

Reference:

[1] Ahmad, 126; classed as Sahih by al-Albani in Irwa’ al-Ghaleel, 7/6.