Income and expenses are always stated in euros in the tax return.
If you had foreign income in a different currency, you can simply convert it into euros. To simplify this, use the annual average of the respective exchange rate. The relevant exchange rate is the rate at the time you received the income.
You can find exchange rates and currency converters on the internet.
The tax offices use VAT conversion rates, which are set monthly, to convert foreign currencies into euros. You can download a complete overview of these exchange rates for previous years from the website of the Federal Ministry of Finance.