Nowhere is it feasible to give a standard leave policy.
Every country has its own prescribed norms for leaves , categorisation and days. You will need to make a customised leave policy for each of the countries and perhaps even states (some states have their own recommendations too!)
The only way one can standardise it is by giving the highest number of leaves of all the 15 countries to all employees! Which would cause a strain on company resources. This call needs to be taken by your company.
Hope this helps.
29th July 2019 From India, Mumbai