Income and expenses are always reported in euros in the tax return.
If you had non-German income in another currency, you can easily convert it into euros. To simplify matters, use the annual average of the respective exchange rate. The relevant exchange rate is that of the year in which you earned the income and for which you are filing your tax return.
Exchange rates and currency calculators can be found on the internet, for example at the Bankenverband or with Forbes:
The German tax offices use VAT conversion rates, which are set monthly, to convert foreign currencies into euros. On the website of the Federal Ministry of Finance you can download a complete overview of these conversion rates for the past years: