In the US I never hear it used in everyday conversation. Only in written forms, and then rarely. In British texts it's more common.

I’m surprised to see others say they rarely use the word fortnight. In the UK it’s still a frequently used term.

The word "fortnight" is a more antiquated term. You will most likely not hear it in a modern context, so it would be better to use "two weeks."

Native speakers are more likely to use in two weeks in casual conversation

It's still good for phrases like "once a fortnight", when speaking to mature native speakers. But children and foreigners might struggle to understand it, so "two weeks" is the safer option for them.