Hello,
why Foreign currency translations cannot be performed in the initial
period opened for ledger?

Thanks, Raghu.K

At the end of the first accounting period you open, income statement account balances would be emptied into the Retained Earning account,
and balance sheet accounts balances will be rolled forward into the next accounting period's balance sheet accounts balances. Because, in the first period you open, there would be no initial account balances in your ledger, you would not be able to do translation in the first open period.
A work around: If your first accounting period is to be JAN-10, open the accounting period for DEC-09. Thus, you would be able to perform
Translation in the first accounting period chosen by your client.
I hope this help! Gavrobere

I have tried to explain the details of the calculations that Oracle goes through for PTD and YTD translations in GL translation videos.
Hasan