If a person gets 2 month's bonus this month, must it be included in the calculation for WithHolding Tax and deducted in this month?

AFAIK you do not need to withold the tax, but the employee has to declare it in his annual tax report. If no taxes were paid, employee has to pay when he gets the bill from the revenue department.

I - as an employer - would withhold the taxes for transparency.

No Bonuses are tax Free !!!!

Just kidding....

If the taxes are not withheld, you will still be libel to pay taxes on the Bonus amount when you file your taxes (it will be added into your total income).

Not sure if it is a requirement to withold for foreign nationals (otherwise couldnt they just skip the country without fileing their taxes?).

At our company, we do withhold taxes from Bonus payments.