You have to pay tax on what you receive from the host family. This means that you have to pay tax on the pocket money/salary and the value of the free board and lodging. When you apply for a tax deduction card, you must know how much you will be paid and how many days of free board and lodging you will receive during the year. See the rates for board and lodging.
In order to find the taxable income, you must multiply the rate for board and loging with the number of days you will receive free board and loging.
More information about tax rules for foreign employees can be found in the brochure Information for foreign employees: About tax, tax deduction cards and the tax return for income and wealth tax.