A single global volunteering policy is the most common design, and it breaks in two directions at once: too restrictive where local rules are permissive, and non-compliant where they are not.
Three areas where the variation is real.
Whether employees can volunteer for their own employer. This one surprises people. In the United States, the Fair Labor Standards Act does not allow employees to volunteer services to a for-profit private sector employer, and the US Department of Labor's Wage and Hour guidance treats the time as compensable hours worked where the volunteer activity is the same type of work the employee is paid to perform, where the employer directs or controls it, or where the service is not offered freely without coercion or undue pressure. A programme that runs internal volunteering days in the US has to be built with that in mind.
Paid leave for volunteering. There is no common standard. Some jurisdictions provide for release or continued pay for specific civic roles, others leave it entirely to the employer, and a company-wide volunteer time off entitlement sits on top of whatever local law already requires. It is worth checking country by country rather than assuming the entitlement is uniform.
Employee data. Recording who volunteered, when, and with which organisation is employee personal data. In the European Union that brings the programme inside data-protection obligations, including retention periods and who can see the record, which is the same problem grant programmes face with applicant data.
Then the practical layer, which causes more disruption than the legal one.
Public holidays do not align. A global volunteering week fixed by headquarters will land on a national holiday in one country, a religious observance in another, and the middle of peak season in a third. Setting a window of several weeks rather than a single date solves most of it.
The structure that holds is one framework with local configuration: the same programme design and the same reporting fields everywhere, with the eligible activities, the approval route and the calendar set per region.
A corporate volunteering software platform is what makes that survivable: one framework across regions without removing local autonomy, forms and opportunities available in the languages your sites actually work in, and approval routing configured per region rather than centrally.
What no platform can do is tell you what the local rules are. That stays a question for your legal or HR counterpart in each country, and it is worth asking before the programme launches rather than after the first complaint.