Is Zakat obligatory for my sister if the value of her money, in dollars and euros, reaches the Nisab when converted to Algerian Dinars, knowing that neither currency alone reaches the Nisab? And if Zakat is obligatory, must it be paid from the same foreign currencies, or can it be paid from her monthly salary?

Your sister is obligated to pay Zakat (definition) on the cash amounts she possesses (whether in dollars, euros, or other currencies) if their value reaches the nisab (definition) of gold (85 grams) or silver (595 grams), and a full lunar year (hawl) has passed since they reached the nisab. The amount due is one-quarter of one-tenth (2.5%). The general principle is to pay Zakat from the same type of wealth on which it is due. However, it is permissible to pay it from the salary in local currency if that is more beneficial for the poor.