Custom and economics make unpaid vacations an unattractive option. Most human resource systems do have "unpaid leave" on the books for when workers have some necessity to take more leave than currently accrued. Under some circumstances a worker could go into a negative leave amount that will be filled in by leave earned later.

One of the issues for a company is that leave is accounted for like an unpaid bill. If employees have a large amount of leave built up it can look like the company has an out-of-balance amount of liabilities.

How is leave accounted for in non-US companies?

It's an option, but entirely dependent on who you work for. I would imagine this would be easy for creative type jobs and not so much for business type jobs.