Fiji and Suriname are two countries that hold that distinction where the original language has not been lost. On a personal note I have met folks from both countries and could never tell that they were not Indians.

These were “girimitiyas” ( English word agreement ) or “jahajis” ( from Hindustani word for ship ) that were transported to work in plantations in these counties in “indentured servitude” - a step above slavery. This surely does not answer why Hindi or some form of it survived in those places while it died out in South Africa and Caribbean where other indentured laborers went. But it provides a background to answer that question. Which I don’t know if OP meant to ask. If you do want to know it I can dig up the old books to answer.

Edit: I was wrong. Some form of Hindustani ( which is the language people generally speak - Hindi and Urdu being two standard versions of it ) also survives in Guyana and Antigua.