Multi-Currency with Google Sheets
bkper does not define units making it flexible to work with any unit (currency).
Rule of thumb, to work with multiple currencies, is not to mixture different ones in one book as balance values will turn insignificant.
Depending on the necessity to keep records in different currencies one can choose for different setups:
If it is relevant to keep track of transactions in each currency create one book for each currency that records transactions in the corresponding currency. Fetch relevant balance values of each currency with the Google Sheet Add-on for bkper to one Google Sheet and consolidate the data into one currency with the formula =GoogleFinance("CURRENCY:USDEUR")
Tracking a Single Currency
With sporadic transactions in another currency, and keeping track of transactions in other currencies is irrelevant create just one book that records one currency. Collect movements in other currencies in Google Sheets and convert them with the formula =GoogleFinance("CURRENCY:USDEUR") into the desired currency and Record Transactions with the Google Sheets Add-on for bkper into your book.
Note: Identify converted transactions adding #converted #rate:xx,xx hashtags to the transaction.